If the subflooring regularly gets wet, it can end up rotting. Depending on the type of subfloor, it could very well be swollen up, especially if there is any luan plywood or particle board there (sometimes used under sheet vinal). Neither one of those handle getting wet very well. If the subflooring is not level, the only way to flatten it out is probably to remove that layer. Depending on the strength and design of your subfloor, you might be able to then tile it, but you would need to know how solid the subfloor is, how deep the floor joists are, how long the unsupported span is, and the thickness and type of subfloor. Regardless, you'd need to remove at least the sheet material. While tile is usually not affected by getting wet, if the subflooring is not prepped properly and strong enough, it won't last either. Once you get a new floor down, then caulking that joint between the floor and the tub should keep it intact. A better shower curtain or a glass door might help as well . Drying that off each time after the shower wouldn't hurt, either. My unprofessional opinion.