'Wow, an underwater version of Silent Hill could be really interesting, couldn't it?'

I keep thinking the same thing, but in regards to Ecco the Dolphin, which is at least not completely a children's game. And there are lots of places in it involving old and abandoned machinery and zombie-looking sharks and dolphins with spikes on them (if you're playing the Dreamcast/PS2's Defender of the Future, anyway). Aliens and trilobites, otherwise.