Swimming is an inexpensive sport as you don’t have to buy any tools and it'd be even cheaper than any other kind of sport if you lived nearby waters \ water area.
I am not sure what is the right noun to describe living beside a (lake or beach or river)? I found some newspapers using "waters" or "water area" but not in this way
"Bodies of water" is the collective term that includes oceans, seas, lakes, and possibly smaller pools of water too, such as ponds or wetlands. However, it might be considered a little formal by some for your context.
From a non-language point of view though, not all bodies of water are suitable for wild swimming. I have taken a cursory look at a couple of websites about outdoor swimming and I couldn't see them using any collective terms. Swimming in lakes is different to swimming in the sea or ocean. The potential problem with using a collective term is that you might be suggesting that one can swim in any sea or lake, ignoring any potential dangers! I live by the sea and the coastguard routinely pull people out of the water who were in trouble.
For the above reason, I would personally clarify by saying:
Swimming is an inexpensive sport as you don’t have to buy any tools, and it'd be even cheaper than any other kind of sport if you lived near a body of water suitable for outdoor swimming.
Some links that may be useful for further research into the correct terminology:
You could use Body of water
Swimming is an inexpensive sport as you don’t have to buy any tools and it'd be even cheaper than any other kind of sport if you lived near a body of water
