I want to make swimable water.
however, the "water" in the brawlbox options is just an effect of water flying around the character when walking.
Is there any way to make true, swimable water?
No. That data is specific to the stages that have 'em, and are stored in their module files.
It might be possible if you make a stage on a stage that has water in it to begin with. But i'm not sure though.