KM: Compared to the way I originally understood your post, the term 'monster encounter' is being interpreted in an increasingly narrow fashion. For example, Exen Trik's post seems to interpret 'monster encounter' to be only applicable to trans-collosus monsters. This makes the idea of 'monster encounter' to be of extremely narrow usage. In the case of the examples he cites, the 'monster encounter' is really a series of individual encounters with various monsters and environmental hazards which share the common 'fluff' of being part of a single mega-organism.
This is far more narrow than I understood your initial post to cover. For one thing, when you said 'sea serpant', I assumed that you were referring to sea serpants generally, and not merely to trans-collosus leviathans say 1500' in length. The encounter as you describe it could basically be run with any sea serpant of sufficient size to plausibly bash the boat that the players are being transported in. For example, you could run basically the same encounter with a large alligator if the PC's where boating in a canoe.
My first complaint can be basically summed up as, "Isn't every monster more interesting as a multi-layered scene, rather than a simple straight-up fight?" Yet, even though this is true, isn't it equally obvious that 'fight' is an option within the scene at some point?
My second complaint is that the less trans-collosus that the monster is, the more obvious it is that at some point you might need traditional stat blocks because the more likely that it is that the monster is within 8 EL's of the PC's and can be challenged directly as soon as the PC's come up with a plan to deal with the unusual nature of the water dweller's attack. More to the point, at some point, even the trans collosus 1500' long leviathan is something that a PC party realistically can effect. A stat block serves as an arbitrator of a parties interactions with the monster. Without a stat block, you are essentially riding entirely on DM fiat, including the DM fiat that 'this monster cannot be effected by anything that the PC's do'. Now granted, the game is not really set up to deal with huge differences in scale like what you describe, and you might need some special abilities to capture the flavor of really really huge well, and it might even be good if there were a set of standardized rules for all trans-collossal creatures (some sort of supplement), but I don't see how stats become obselete just because things get big. At some point, isn't the DM going to have to make calls like, 'Is the DR of this creature 30/- or 50/-?' 'Does it have 10,000 hit points or 15,000?', 'How fast can it move?' and so forth. The alternative is to just wing it, yank the PC's chain, and risk (as you put it) 'abusing the PCs'.
I'm not very fond of the notion of encounters as putting the PC's on a railroad, which is what a statless encounter seems to imply.
And you seem to agree, since you say, among other things:
For stats, I'd basically need some sort of attack rating for the DC of the saves it forces, some sort of damage rating (that I can halve for being behind a ship, or double for being swallowed, etc), specific hp and AC and actions (we don't need it's whole AC, just the AC for stabbing it in the eye), how to escape it's gullet, maybe a swim speed, a strength rating, how it "rammed" the ship....
Isn't this going pretty darn close to having a stat block?