Oceanside is a town located right on the Pacific Ocean so there are plenty of water sports offered in this area. They say that surfing is pretty good up this way, but for those who don't surf I'd suggest renting a jet ski and ride the waves with that. It's sort of the motorcycle of boats and can be great fun and thrilling.
You should know that this is an expensive ride even as a rental, so if you didn't bring your own jet ski and it is too much for your budget try renting a kayak instead.
Equipment: A Jet ski and nerve!
At the Oceanside Harbor you can make arrangements to go on a Whale Watching boat tour, deep sea fishing adventure or take sailing lessons.
Personally I would choose deep sea fishing - I didn't discover it, yet :-). Did you?
Equipment: There are also sailboats, kayaks and Wave Runners available to rent.