Hmm... I would try Hippocampus ingens. A bunch of divers keep finding them off the coast of La Jolla. Make sure that the lagoon doesn't get too hot in the Summer. You should think about getting some of the invasive sargassum that's all around Catalina, the seahorses would love using it for a holdfast... oh yeah and Catalina gobies would make very good lagoon mates for them!
I was just getting my scientific diver cert. in Catalina, you are so lucky to live there!