I've heard that pulling a wool sock over the bottle can help delay water freezing. The problem is that the ball nozzle on a bottle can freeze long before the water does.
I'd switch to water crocks for the winter. There area heated crocks available, but depending on how many rabbits you have, that could get costly. Instead, I've heard that most people get two crocks per rabbit. That way, you can give fresh water in the morning, then switch the crocks in the evening- fresh water in the new one and bring the frozen one inside to thaw. If needed, you can switch them out more often of course.