Oooh now, doesn't it depend what era the engine is from?
It isn't the gravity per se, it's to do with the water. What's happened is the nuts/spanner/etc are naturally drawn towards the gravitons in the water. By putting your boat into water you've created a natural gravity well where not only is the nut attracted by normal gravity, there are additional forces from the surrounding water.
If there should be a large metallic object in there like an engine, that increases the attractive forces