I work at a coal fired power plant that takes its cooling water from the sea. We have very strict regulations about the state that this water and its inhabitants is in when it returns to where it belongs. It must not be contaminated, a very high percentage of the fauna must be unharmed and it may only be 7°C warmer than before, but no hotter than 30°C. The only reason you should not swim in this cooling water is the flow that could carry you from your desired way.<br>The ash of course is another topic though. You are absolutely right about the contamination with heavy metals, and you surely wouldn't want to have that on yourself.