Skyrim on PS3 is notorious for water-related freezes. The engine's water shaders, reflection rendering, and cell loading push the console's limited RAM (256 MB system / 256 MB video) to the brink. Here's how to diagnose and fix it.
If freezes persist across clean saves and vanilla game, your PS3's RSX (GPU) or RAM may be failing. Contact Sony repair or consider a slimmer, cooler model (CECH-3000/4000).