That's just your head getting in the way. Its knowing that you HAVE to carry the ball a certain distance that makes you think you should swing harder or change something. Getting thoughts like these out of your head is one of the hardest parts to playing good golf, as I know many players who strike the ball well but always get in their own way when trying to score.
I find it helps to focus on where you want to hit/miss the ball instead of where you DONT want to miss. When you are saying "dont hit it in the water" your brain is thinking water and guess where the ball will probably go? Water!!! Try to be as positive with the 'self talk' as you can, saying "just clear the water and I'll be okay" or "if I miss anywhere its going to be long". If you are worried about a carry, instead of trying to swing harder, take an extra club and trust your swing. You should also be looking for the proper bail out zones, so if you do happen to miss the shot, you are aiming in such a way where it won't cost you too much. (short into water vs over into bunker)