Swimming is the main exercise that I do also, and I do water exercises as well. I walk sometimes, but weather is an important factor in being able to be outside and walk, so it is only when the weather is not too hot, cold, or rainy that walking is possible for me. I swim at the fitness center, which is inside, so I am able to do that anytime that I want to.
I think that as long as a person is doing some kind of exercise that they enjoy, that is the best exercise that we can do, because it is beneficial to both our body and our mind.