How to fix a squeaky floor below the carpet

You can make the floor water resistant by varnishing it. Two or three coats should protect it from most damage, especially if you lift the affected tiles straight away. Most damage is done by the prolonged contact of the water with the floor. Are you expecting a lot of spillage?

This does mean that you might want to consider not sticking the tiles down or only using a very light adhesive. If the tiles are well fitted and cover all of the floor area there shouldn't be any lateral movement in the tiles themselves.

The joists should be at the squeak, you just need to determine the direction of the joists. Typically, this is the shorter distance between exterior walls, unless there are interior load bearing walls used to break up the span. Some stud finders have a deep penetrating settings that might find a joist through carpet. And you can also try finding the joists in the ceiling below and measuring the distance from a common wall.