Your meant to stand between the walkable tile and the water to reach the hammer pegs/clam things. In 1200 you simply can't walk up between the tiles, it just acts like a wall. (2.10 quest)
1200: Can't use a ladder over half-walkable and half water tile.
Confirmed. You're supposed to do this:
...and pound the post to the right. Works in 2.10, broken/changed in 2.5.
Hey, you have the red armor -_-
Question: do you have the Flippers? It appears that you can still do it if you don't have Flippers.
Yes, you have the flippers. The only way to solve this room is to swim across the pool from the top door, use the half ladder trick, and pound that first post/mole thing.
EDIT: Oh, and let me make a statement that since I'm done quest making until... well, 2.5 is finished, I'm readily available to beta test like a madman. First I think I'll replay through my Link to the Heavens (lots of fun little tricks, heh) to check for compatibility. Then I might actually crack open the 2.5 ZQuest for the first time in... well, a long time.