OP, if your hospital allows it, I would just let them know you're planning on it and then you can always change your mind, y'know? I'm due in a week and my midwives know I am planning a water birth. I will have one of the two water birth rooms at my birth center/hospital, but there's nothing that says I HAVE to get in the tub or stay there, and I'm figuring I will get in and out as I desire.
I'm planning a water birth because I know how soothing water is to me, and I know how much it helps me with pain. I've dealt with terrible periods since I was a young teenager and the ONLY thing that helps at all is being in hot water. I really think I will like it, but like I said, I know if I feel like getting out and laying down or squatting or walking, or whatever, I have that option as well.