After this he equipped the Piraeus, because he had noticed the favorable shape of its harbors, and wished to attach the whole city to the sea; thus in a certain manner counteracting the policies of the ancient Athenian kings.[Plutarch, Lives, Themistocles 19.2-3]
For they, as it is said, in their efforts to draw the citizens away from the sea and accustom them to live not by navigation but by agriculture, disseminated the story about Athena, how when Poseidon was contending with her for possession of the country, she displayed the sacred olive-tree of the Acropolis to the judges, and so won the day. But Themistocles did not, as Aristophanes the comic poet says, ‘knead the Piraeus on to the city,’ nay, he fastened the city to the Piraeus, and the land to the sea.
Themistocles (along with Pericles) was one of the architects of Athenian greatness. He is also (along with Pericles) something of a villain in the Platonic dialogues, and the above passage by Plutarch captures, I think, why this is, since Plato sees Themistocles as perverting the true nature of Athens. In the next sections Plutarch will note that the ancient Greeks thought of maritime empire as the "mother of democracy", because it puts the real power in the hands of those who control the ships; and I think the argument can be made that Themistocles' subversion of Athenian principles connects to Plato's skeptical views on the stability of democracy.
The place one can see the thematic opposition play out most clearly, although without any names, is in the Atlantis myth of the Timaeus and the Critias, in which the island is the symbolic representation of the ideal of maritime empire, which is opposed to the autochthonic, and thus land-focused, Athenians. The sea as a factor in Athenian corruption comes up elsewhere in the dialogues, e.g., in Laws. But it is in the Gorgias that Plato is fairly explicit about what, precisely, he sees wrong with the Themistoclean sea-dream.
Themistocles was the first politician fully to understand the implications of the overthrow of the Athenian kings and their replacement with the Demos: namely, that anyone could have any power that they wanted as long as they could convince enough people to give it to them. He is also very recognizable as a politician in our sense of the word. He moved to the poor part of town, playing the part of a man of the people, going from door to door and asking the poor citizens of Athens for their assistance, being careful to remember everyone's name. People, of course, loved it, and thus Themistocles built a constituency; on the basis of that, he campaigned for office. Being an excellent speaker, roused the crowds in his favor again and again. It is thus not surprising that Gorgias mentions him as an example of the power of rhetoric (455d-e):
GORGIAS: Well, Socrates, I'll try to reveal to you clearly everything oratory can accomplish. You yourself led the way nicely, for you do know, don't you, that these dockyards and walls of the Athenians and the equipping of the harbor came about through the advice of Themistocles and in some cases through that of Pericles, but not through that of the craftsman?
SOCRATES: That's what they say about Themistocles, Gorgias. I myself heard Pericles when he advised us on the middle wall.
Themistocles is certainly in mind when Socrates and Polus argue over whether one wants to be able to do whatever you like, "whether it's putting people to death or exiling them, or doing any and everything just as you see fit" (469c). One of the major early highlights of Themistocles' career was his convincing the people to ostracize (temporarily exile) his major political rival, Aristides. But another possible sign of it is when Socrates rejects Polus's idea by using the example of the "marvelous tyrannical power" of a dagger (469e):
On seeing it, you'd be likely to say, "But Socrates, everybody could have great power that way. For this way any house you see fit might be burned down, and so might the dockyards and triremes of the Athenians, and all their ships, both public and private." But then that's not what having great power is, doing what one sees fit.
We see a reverse of this in the life of Themistocles. At one point, it is said, Themistocles came up with a plan to guarantee Athenian sea superiority: he would burn the combined fleet of Athens's allies, which at the time was wintering at Pegasae. He told the Athenians that he had a plan that would be extremely advantageous to them -- but he couldn't tell them what it was beforehand because it needed to be done with secrecy. And the Athenian Assembly, in a moment of wisdom, told him to tell the plan to Aristides, and if Aristides also thought it was a good plan, he would be fully authorized. When Themistocles told Aristides, Aristides replied to the Assembly that there was no plan that could be more advantageous to the Athenians, and none that could be more unjust; so they refused.
Themistocles comes up again explicitly in the discussion between Callicles and Socrates. Socrates asks whether the oratory directed to the Athenian people is done with regard to the best; that is, whether such political oratory is concerned with making people better or gratifying them like children. Callicles responds that it is too simplistic to put it this way, because some do and some don't. Socrates concedes the point, and says that it suffices for his purposes: there is flattery, which is shameful, and there is encouragement to virtue, which is admirable. But has Callicles ever seen the latter? Callicles responds that none of their contemporaries seem to be such; and Socrates asks whether this is also true of former times. Callicles mentions Themistocles, Cimon, Militiades, and Pericles, all of whom were major players in Athenian military greatness. And Socrates' response is unequivocal (503c-d):
Yes, Callicles, if the excellence you were speaking of earlier, the filling up of appetites, both one's own and those of others, is the true kind. But if this is not, and if what we were compelled to agree on in our subsequent discussion is the true kind instead--that a man should satisfy those of his appetites that, when they are filled up, make him better, and not those that make him worse, and that this is a matter of craft--I don't see how I can say that any of these men has proved to be such a man.
Socrates will later press Callicles on the point again, and when Callicles insists that these Athenian heroes really did make the city better than it was, Socrates replies that if this is so, then when (say) Pericles started out, the people should have been worse, but as time went on, they got better. But, Socrates notes, as time went on the Athenians got tired of all of these people: Pericles was convicted of embezzlement (although he was later restored to office), Cimon was ostracized, Miltiades narrowly avoided being put to death, and Themistocles was ostracized and later, under encouragement from the Spartans, they tried to summon him to trial, probably on trumped-up charges, and he fled for his life.*
Callicles has throughout been arguing that someone who pursues philosophy rather than oratory can be charged on false charges and unable to defend themselves, so although it is not emphasized by Socrates, it is very relevant to the discussion. People like Themistocles work by flattering the people, and it is no more surprising that this makes the people worse than it is surprising if you became less athletic because you got nutritional advice from the doughnut shop, or that people become sick if they are given incentive to do things that are bad for their health. To be sure, people praise Themistocles for his achievements. But in reality they have sickened the city and erratic behavior is inevitable (519a):
For they filled the city with harbors and dockyards, walls, and tribute payments and such trash as that, but did so without justice and self-control. So, when that fit of sickness comes on, they'll blame their advisers of the moment and sing the praises of Themistocles and Cimon and Pericles, the ones who are to blame for their ills. Perhaps, if you're not careful, they'll lay their hands on you, and on my friend Alcibiades, when they lose not only what they gained but what they had originally as well, even though you aren't responsible for their ills but perhaps accessories to them.
Of course, this does not mean that they will not harm Socrates, either. But if the leaders of the city are already sickening the city, its behavior will be erratic regardless; the question is, what politics makes the city healthy rather than sick? And, of course, the answer Socrates gives is that he is the only one, or perhaps at best one of a few through history, who practice the "true politics", the kind that aims at what is really good for people rather than what they merely happen to like.
* It is worth keeping this in mind when considering the argument Socrates gives in Plato's Meno that virtue cannot be taught because Themistocles, as a virtuous man, would have taught his sons more virtue than they show -- we should be wary of the ironic edge of the argument.
Quotations are from Plato, Gorgias, Zeyl, tr, in Plato: Collected Works, Hutchison, ed., Hackett (Indianapolis: 1997).