Oceans form at elevations of less than 100 units. All biomes besides mountains form at elevations between 100 and 299 units. Mountains form at elevations between 300 and 400 units with peaks forming at greater than 380 units. Rain and drainage affect which biomes form in the 100-299 unit range (which means that unless you nix the minimum square number when you mess around with rain and drainage you'll get an infinite reject loop). Numbers can be found at
this thread, along with some info from Toady on some specific details. At first I thought that lakes would also form at the 0-99 range, since the OP in that thread just said "water", but since lakes form even without the elevation range being available that doesn't seem to be the case (suggesting instead that it is based on relative elevation or the convergence of rivers or something).
Rivers seem to mostly care about relative elevation more than oceans, though, as you can quite easily create a world full of rivers but devoid of oceans, and lakes are placed after rivers.
Your note about rivers not seeming to "grow" with more oceans to drain into is interesting, Nonanonymous, are you checking the water on the region screen? And, if so, are you sure you don't mean stream as opposed to brook (as brooks are not visible on the region screen)? It might be some interaction between the elevation variables, meaning that having higher elevations (as opposed to the very low ones needed for oceans) encourages larger rivers, or possibly something in the shift of elevation from high to low...
I don't suppose you could post the parameters you used? You've gotten me curious, now.
(also, am I getting to analytical in my tone of voice? It bugs me when I do 'cause I used to sound like a brat trying to sound smarter than everyone in the room. I don't know whether it's all the reposting of the information that I've been doing recently or some sciency stuff I've been thinking about in real life but today I've been typing differently and it bugs me.)