I feel like this is genuinely one of the easier things you could get him to believe

one of my favourite things when looking through historical perceptions of the natural world, etc. is when there's enough context on the general frame of reference to show how these things would have seemed plausible to people of the period.

like when I was younger it was easy to rest on the cliche that people were Dumber during medieval times in some vague but essential way -- but these folks had the same brains that we do, and were forming an intelligent and functional understanding of the world at large based on the limited information available to them. trying to put yourself in that mindset, where there are all these animals you've never seen up close but which seem to follow an internal logic that you can extrapolate, is a lot of fun!

(I'm being brief and reductive but of course there's a lot more of this kind of philosophy in european medieval bestiaries about the animal kingdom existing as an allegory or lower-tier expression of the ordering of human kingdoms and other concepts within them -- it feels like a very comfortable sort of worldview, where conceptual things are made self-evident in how nature presents itself.)

As Dominions tells us over a dozen unit descriptions, "Life under the sea corresponds in many ways to life on land."