The specifics of it are up for discussion. I didn't want to set everything in stone before talking to the person who wanted to play Caspian's familiar. :) As it is, Caspian doesn't quite remember why he went out to the sea, just that he felt drawn there, and you don't ignore your instincts. Reading all those suggestions, I think the one that would fit the most would be that the spirit guide didn't understand the currents/ need for oxygen/ human fraility. And maybe that was the first thing Caspian learned to do with his water affinity - how to not drown. I sort of like the idea of it being a pull so strong that he can't deny it, because a lot of Caspian and his ability (and the element of water in general), is primordial and extremely uncomplicated. You cannot command the sea. Whether or not your character feels guilty is up to you, because Caspian doesn't feel like it was a bad thing. In the sea, he felt liberated.
The familiar could have been waiting for Caspian, or maybe he was waiting between Caspian and his cousins to see which one he felt most compatible with. Caspian's family hasn't had hereditary witches in generations, but since he almost drowned, two other cousins have shown the same affinity.
I have no problem if you want to play both characters. It's entirely up to you. I'll try to steer away from the same themes as the other character, but that shouldn't be a problem since they're so different and don't take the same route with their magic.