putting in tub surround with a window
Best thing to do is get rid of the window totaly, reframe and just have a transom window. or add a solor tube. It's amazing how the whole room can light up with one solor tube. You can even get one with a light in it to use at night.
If you insist on keeping the window get a vinyl replacement window and replace all the trim with vinyl and seal all the trim behind it with silicone caulking.