not quite the same set of muscles believe it or not
surfing works the triceps, muscles at the top/outside/front of the chest (not an anatomist) and lower back quite a lot more.

edit: my view is that I swim to work most of the appropriate muscles, and improve the others by surfing. It suits me, but you can't help but curse my mate when he can paddle twice as powerfully as I can.
( Vipros. clever got me this far, then tricky got me in, Fri 7 Jan 2011, 10:58, 1 reply, 15 years ago)

so does swimming. Especially on a float.
I do see your point, but there's no gym exercise that would be any better, it's just the easiest substitute I guess. Surfing's a rare example in that it's tricky to train by just doing it, though.
( the mighty badger Aphrodite, on a bar stool, by your side, Fri 7 Jan 2011, 11:01, Reply)

you are right, and using a float is something I've considered
but only since having access to a private pool.

you can build the muscles, especially arm and chest, in the gym, but not using them with the appropriate motions.

Core strength is a big thing, but that is very easily done outside the gym.
( Vipros. clever got me this far, then tricky got me in, Fri 7 Jan 2011, 11:05, Reply)

Core strength is there in pretty much any sport, though
is surfing the only thing he does?
( the mighty badger Aphrodite, on a bar stool, by your side, Fri 7 Jan 2011, 11:08, Reply)

surfing is the only thing there is!

also, yes, well, he plays double bass as well. That requires quite a lot of upper body and arm strength.
( Vipros. clever got me this far, then tricky got me in, Fri 7 Jan 2011, 11:13, Reply)