I used to soak mine in the bathroom sink, so I would empty and refill. Some will put the soaking tub part way under the basking bulb. Making sure that not the whole container is under it, so as to not over heat the tort. Some will set the tub on top of a heating pad. I would not use an aquarium heater, unless it's not made out of glass and your tort can't step on it.
1. Soak in their closed chambers.
2. Put the tub under, or partially under, a heat lamp or CHE. MONITOR VERY CAREFULLY!
3. Soak them outside in the sunshine on a warm day. MONITOR VERY CAREFULLY!!!
4. Refresh the soak water with warm water every few minutes. This is easiest with two tubs. Pop the tort in a clean tub with warm water, and after a few minutes, put some warm water in another tub and switch him over. Repeat as needed.
I don't worry about it. I position the soaking tub at the edge of where the light shines down into the habitat and leave them there for about 15 minutes. If it gets cold, my house is usually around 75F degrees or so, and the water can't get any colder than the room temp is.
I've tried 2 ways:
The two tub method that Tom mentioned.
And putting the tub on a waterproof heating pad. Unfortunately I have to change their poopy water so often that I've yet to find out if the heating pad method actually works lol.
Actually, the water certainly does get colder than the room temperature is. Anything damp or wet is chillier than anything dry. If I measure a damp or wet area (including a pan of water) on the cooler side of the enclosure it is usually 4 to 5 degrees cooler than the dry parts on the cool side of the enclosure.
Anyway, if it is a closed chamber enclosure the water is going to stay warmer than if it is an open topped enclosure. I keep water dishes either near the heat bulb or directly under a low wattage heat bulb.
My tortoise will actively seek out his drinking pool and just sit in it for ages, and that's just sitting at random temps because I don't regulate it, plus I bath my tort for 10 minutes in a bathtub so temperature is never really a concern because he isn't there long enough for it to become an issue