Sauna and hot tub are used to rburn off lactic acid built up from your muscle, I use hot tub as often as possible. Both will dehydrate you severely, given you are only suppossed to stay in them for 15 minutes at 104 degrees. Although, Hot tubs are proven to burn quit a lot of calories. Given that your temperature is roughly 98.6 degrees, hot tubs usually at 104, so your body has to work over time to keep it lowered. Just make sure you hydrate your self back up.
****Don't worry about water weight, unless u are competing and near competition.****
not smart idea if bulking, to stay in there too long