Items sinking and making retrieval of them hard or impossible makes sea journeys something to plan and makes them more intense. That said - I wouldn't mind a a potion to make you heavier than water. This would allow you to more easily bring shoreline items back, but it would also drown you if attempted at sea.

I haven't had any issues with enemies running in water - they can move under water, but I think they should be able to. I've never seen any enemy run 90 degree hills (cliffs) in full speed either. I doubt they can.

I've played the game enough to say I don't believe troll raids need nerfing. It's up to the devs, but if they nerf all hazards in the game, there will be little challenge left.