A few weeks a go myself, the girls and some members of my family ventured all the way to Penzance to the Sea Pool there. It is an outdoor (unheated!!!!) pool which is full of water from the sea. There is a large pool and a baby pool. It has a really retro feel to it. It costs £5 to get in and the girls were free. Under 4s are free but they must be accompanied by an adult on a 1:1 basis.
The pool is freezing but once you get in it isn’t too bad – pretty much the same as the sea. I’d say wetsuits are a good idea for little ones in particular. The best days to go are definitely hot days that you can soak up the sun on the side in between dips. The cafe is also great!
It’s definitely worth a visit!