I have a lush balance board.....MSW sell them (or used to).
NIce bit of kit and good fun to spend some time on.
They are fun and can stregthen up leg and core muscles a bit and help with balance if you get into the I'm riding a barrel and grabbing the rail pose (
) ......but they are fun most of all. I doubt it has done too much for my surfing if I am honest.
If you want to work on core strength an easy way to do that is to do is a plank and side plank each day....