To prevent water from getting behind or underneath your tub, run a bead of caulk where the bathtub meets the walls. If water can enter the space between the wall and bathtub, mildew and mold can form and rot your drywall. Don't forget to caulk around the handles and spout, where the metal meets the tub surface. Look for mildew-resistant caulk to prevent the unsightly organism from growing on your caulk.
Caulk provides a seal and protection from water when applied around your sink. If the sink is connected directly to the wall, apply a line of silicone caulk in the area where the sink meets the wall. Water could fall in the gap and damage the cabinet below it. If the sink doesn’t connect to the wall, you may have to apply caulk to the area in which the sink bowl meets its stand. In addition, if the sink drops down into a counter, caulk their meeting place as well.
Apply caulk to the areas in which your bathtub, toilet and sink touch the floor. Doing so will prevent water from entering the space under your floor — an ounce of prevention, you know. A rotten subfloor is not an easy fix, especially if you have only one bathroom in the house.
If you have tiles on your floor, you likely need to caulk where they meet the wall. For those on the walls, caulk where they meet the sink, toilet or tub. Caulk can also help to prevent water from getting underneath the tiles.
Ensure the places you plan to apply the caulk are clean or the caulk may not work properly. A bit of rubbing alcohol applied to the surfaces to remove any residues should do the trick. When in doubt, if water can get behind or underneath or in between something, caulk the joint or transition.