I remember having rabbits in the water with me when I was younger. They can swim, but not very well. It's harder for them to hold their heads above water and they tire quickly. I would imagine there are exceptions to every rule, and you will probably find a rabbit who may take to the water like a duck, but this is probably rather rare.
Another thing I recall is that the water can get into their ears easily, and could cause blockages and/or infections. I would definitely discourage anyone from trying it with their rabbits (even though the pics of rabbits swimming are very cute).