Resistance pools
 

Anyone tried these? If you've never heard of one it's this kind of thing.. Endless Pools Swimming Machines | Swimming Pools, Indoor Exercise Pool where you swim against a current. I know some gyms have them and you can have them fitted into your own pool.
I like swimming, and it really helps when I'm hurting after boarding but I think I end up doing the same thing every time so not sure it's doing me much good. This sort of thing looks like it's more of a challenge.