Noob question

Hi! So whenever I play SP maps (usually 2v2), and there's water, I'll usually build castles in strategic locations to gain control of the water. Now, I understand that a human (unlike a stupid AI) is unlikely to throw fireships and galleys at a castle, but it seems like that's a strategy that could be used to gain control of a certain area (because no one's gonna sail their ships into range).

But I almost never see this when the pros play. Why is that? Is it because it's always more advantageous to build an inland castle, and instead try to maintain control of the area of water a castle would be protecting by building docks and ships. And if so, why? And if there's another reason, what's the rationale?

Thanks!