No, hot water bottles are dangerous.
They can burst
They can be chewed and then burst
They don't stay warm/hot for long and then go really cold if outside (it ends up being full of cold water after all)
They have the potential for being made too hot
Without stating the obvious, water isn't solid and if your bunny decides that the floor of his bed is moving in an alarming manner he won't go there
As already mentioned, you are much better off buying a Snugglesafe heat pad as these are especially designed for animals and usually have a cover. Yes, they generally are quite hot to the touch but the cover is amazing and brings the whole thing to a lovely temperature. Even when these have gone cold, they are not stone cold like cold water in a water bottle would be.
Don't buy the first one you see as they vary quite a lot in price (but take into account p&p too) so have a look around the various websites before committing.
Personally I would bring your bunny indoors for a good few days (mine were indoors for almost 2 weeks after their spay and that was on the advice of my vet and they were done in November too) and I had no problems at all with them. You have to make sure she can't stretch as she risks ripping open her wound and being outside in her normal shed/hutch/etc she runs that risk quite highly. Bring her in if you can. If you can't, then make sure she can't get upstairs in her hutch as even using the ladders will put pressure on her wound and cause it to re-open. It was a right nuisance having my 2 girls indoors, but well worth it.
I hope Luna heals well and quickly