When entering the Water Gate level, the frame rate drops drastically (like to 2 or 3 fps, when before that it was something like 60 fps). This is the first time this happens in the game for me. Any suggestions? Reducing the detail doesn't do a thing.

Hmm, I didn't have any problems like that. Without more specifics, the only thing I can suggest is reinstalling the game, reloading save games, or loading a previous save to over-write the new one.