If it's just for a few maps, then try the following:

1. Go into Tilesets, increase the Array Size by one.
2. Copy/Paste the tileset you were using into the new slot, rename it to whatever you want.
3. Alter the water's passiblity in the new map to O.
4. Change the maps you want to have to this new tileset.
5. Make a bunch of "clear" events, with the event layer being "Same Level As Hero". Use them as a border just inside the water's edge. This will prevent the hero from walking in, and the fish from swimming out. (Copy/Paste works great here)

Hopefully your map doesn't have too much of a border, otherwise step 5 may be a bit time consuming.