Well, as the post says, the source is Thucydides (book 4 I think?). This is one of my favorite incidents as well, and ultimately led to the capture of 400 Spartans (including Spartiates) and the Peace of Nicias - though it wasn't these marines that captured the Spartans, it was Messenians. Demosthenes was (in my opinion) an especially talented general. The fort he established was in Messenian territory and acted as a refuge for helots trying to escape Spartan domination, and so threatened Sparta's economic system. The Spartans negotiated a peace with Athens (to get their men back) that lasted until Athens decided to go after Syracuse. The armor of the captured Spartans was put on display in Athens.

These marines aren't particularly elite. A tririme had probably 170-something oarsmen and 5-10 marines. Demosthenes wasn't even an official general at the time and convinced an admiral to give him 5 ships to take advantage of an opportunity he noticed (taking Pylos) - five random ships. He managed to get reinforcements from both the nearby Athenian fleet and Athens (and the local Messenians), and the Spartans screwed up by getting their guys trapped on an island, but this wasn't about super elite training. It was a smart general being lucky by applying pressure to his enemies' cultural insecurities, backed up by a big military machine.

The Spartans just got outmanuvered here.

The Peace of Nicias fell apart. Demosthenes went on to get himself executed as a POW in Sicily some ten years later. In the end the action at Pylos doesn't seem to have mattered to the outcome of the war, as far as I can tell.

Oh Jesus, I don't remember when I wrote that. Must've been in my Demosthenes fanboy stage. Ugh, the rhetoric. As /u/LegalAction notes, the incident in question isn't about Demosthenes bringing a super-duper elite force to meet the Spartans. The marines at Pylos were just from five random ships, and Demosthenes simply out-generaled them. There was nothing particularly special about the marines he had, at least compared to the other marines.

The marine force in general is a bit hazy. I agree with Kagan's estimation (which I think comes from Hammond's earlier one) that the front-line troops that Thucydides mentions must mostly have consisted of marines and garrisons. Thucydides breaks down the Athenian land forces at the beginning of the war as consisting of about half front-line troops and half reserves (I think Hammond calls them second-line troops, but they're basically reserves only called up for large, pitched battles). The front-line troops were at arms for some time, probably years at a time if they were in isolated garrisons and on board patrolling ships, and represent a well-drilled army, though how professional they were is debatable. Clearly they were well-trained and a good match for many Spartan forces--the land engagements in the early part of the Peloponnesian War were mostly fought by marines, often heavily outnumbered (Demosthenes' Acarnanian campaign is a good example, as are the attempted marine landings before Delium). So they represented the bulk of the front-line force of the Athenian army. But large campaigns, and in particular the battles fought after the early part of the war, were composed of armies in which the marines were only a tiny fraction. And the glamorous successes of the marines during the earlier part of the war are as much due to the superior tactics of the leaders commanding the contingents--Demosthenes in particular, Thucydides says, used a number of very unconventional tactics to even the numbers (tactics usually well-suited to the marines' function as raiders and so forth, often relying on ambushes and stuff).

I tend to use very very inflated rhetoric (too inflated, in that post. I try not to write like that anymore shudders) when speaking of the marines for a purpose. The purpose of the rhetoric is to show that there were plenty of other highly-trained contingents contemporary to the Spartiate Similars. The sort of hero-worship centered on the Similars that's so common in popular culture really irritates me, which tends to make my rhetoric in those posts a bit anger-driven too. I've found that much of the hero-worship is due to rhetorical expression, which I (at least at one time) figured I could counter--certainly I've found it effective, though maybe in the wrong ways. The post in question was less about the actual qualities of the marine force and more about fracturing the Spartan Mirage, and I apologize for the confusion. I've learned how to write better, I promise :3

Anyway, what /u/LegalAction says is pretty accurate, and I thank him for cutting through my rather sophomoric rhetoric (I really can't apologize for the rhetoric enough, I was in a very rhetorical phase a while back. It's all the Cicero...). We mustn't think of the marines (or the Similars) as being Master Chief-esque super-soldiers who could beat any foe, either through inherent superiority or training. The permanent hoplite forces of the Athenians (which also includes guys like the Messenians at Naupactus, who performed admirably during the war) were just as formidable as the Similars, and could be extraordinarily effective when using tactics suited to them. The same thing could be said of the Similars--they performed great at Mantinea (albeit against an army that hadn't fought a war in like a century), but suffered against the unfamiliar tactics employed against them in Acarnania or at Pylos. In either case neither contingent was large enough to affect the war permanently. The Spartiates only came out for battle on a handful of occasions in any significant numbers, and the marine force was simply too small and was spread out across the entire Aegean (after all, their main duty was aboard ships patrolling the Aegean). We must avoid thinking of this as being a sort of Deadliest Warrior kind of thing--the successes and failures of a fighting force are very dependent on the circumstances in which they perform, and I don't buy military historians who try to make the argument that there's any way to even approach a quantitative evaluation of how "well-trained" or "well-prepared" a force is. Could the marines give the Spartans a run for their money? Sure, they did several times. So could many of the Athenian front-line troops. Were they elite super-soldiers who could've won the war? No, nor were the Spartiates. Both were relatively small elements within a much larger force

EDIT: I dunno where I got 800 marines from. That doesn't seem right.