In addition to all the great advice above, you can invest in either a heated water bottle or a continuous flow system.
This Bottle has a thermostat built into it so it only uses power when the water gets close to freezing. At $26 it is reasonable for just a few rabbits.
A CFWS has the benefit of never needing filling and not freezing in most human inhabitable places. Depending on the distance between a heated structure and the hutch a CFWS would cost less than $200. I have a promise to publish my plans here at RT, but I have yet to get time to build mine. Here are the parts:
1) a reservoir - a 5 gal bucket would do
2) a float valve - there are many styles, there is one for cattle that looks like a pint bottle
3) an RV pump - runs on 12v and has low pressure
4) PEX and fittings
5) valves - used to cut off flow when doing maintenance
6) nipples - of course!
7) tap for the nipples
The reservoir goes inside the house or heated structure. A water source is connected to the reservoir via the float valve. This is what keeps the system full forever. Using on bulkhead fitting at about the half height of the reservoir, stretch a run of PEX out to the hutch. That is the return side. Use another bulkhead fitting near the bottom of the reservoir and connect the pump. From the pump stretch another run of PEX out to the hutch. Run PEX through the hutch and add nipples as needed. Turn on the pump. Rabbits now have a constant supply of water that won't freeze unless you live at one of the poles. Bury the PEX deep enough so you won't damage it with daily traffic. Don't worry about getting it deep enough to not freeze, it simply won't while the pump continues to run. Added benefit is that in the summer the water is always cool for the bunnies too!