Thanks to John Walls interview, we know there are going to be water rides in the base game (he said not a lot, becuase they want to get them right), so thank god for that, they are probably already already working with the water physics.
What im pretty sure that is going to appear are the typical vanilla RCT rides:
-Log flume
-Shoot the chutes
-River Rapids (the thin version)
-The floats coaster.
-Some type of swans, tow boat or water transport rides (if those exist, we still dont know about them).
Those are the staples of a real theme park, the minumum a game will need.
Now you can start adding a little to expand the water rides flavour, I really hope they add (and they are probably going to appear, but who knows):
-River Rapids (the wide version that appears on Soaked for the first time, as I think is more regular on theme parks than the thin version).
-Some extreme version of the shoot the chutes.
-The really special pieces of all this rides. Log flume going backwards, whirlpools, elvators, circle riverapids station (RCT never got right this ones, becuase they were too difficult to implement)...
And something really importat that I dont know how hard would be to implement, is that all this rides need to have special rails that use already made bodies of water, just like in real life, instead of only rails totally separated form the waters you can create (like we always had in RCT).
But dont be expecting the whole collection of water parks and slides that appeared on Soaked, because thats probably not going to happen until a expansion comes along.