That sounds perfect to me. Caspian might feel a little overwhelmed at first, but he would love that. I actually thought about it more last night, and had an idea to pitch: Cas nearly drowned when he was a kid, but came back much stronger and powerful magic-wise. What if he wandered away from the shore because he felt a pull from the sea, and that pull is the familiar (who would be an unknown spirit guide at the time)?
A marine iguana was what I was originally going for, so that sounds excellent. They can both commiserate about being away from the sea.
I have zero issues with him being the human face of the familiar at all. The PB is entirely up to you! Cas would feel close to him regardless of the face.