It seems crazy but these people lived their entire lives on small islands and had nothing else to do but master what little information surrounded them.

I've heard stories of Islanders that could detect different types of fish from surface wave patterns.

I get their mastery of the environment they knew. Say, navigating between inhabited islands, or doing long fishing trips, or that kind of thing. What I don't get is how they bumped into stuff like Easter Island for the first time. The clouds mentioned in another comment seem kind of plausible, but even still, it seems pretty amazing.

This isn't as difficult as you think. When you know an area well, you know what species of fish are likely to be around - down to specific habitats like rocky vs sandy coastline, and even particular arrangements of rocks or sand formation. After a while you notice their behaviours and specifically behaviours distinct from other species (e.g. bottom feeding, surface hunting, swimming style etc). So it's more of a combination of inputs that gives you an educated guess as to the fish you can't see, based on what you know about the area as well as fish behaviour. Anecdotally, I have an avid fisher of an uncle (more than one actually...) who is able to tell what species of fish he's hooked before he can even see it.