Sailing near a coast and then in the ocean near swamp/plains islands they seem to always spawn like clockwork. they also spawn in roughly the same spots so it's probably time based. You can also find them in any time of the day in any part, i found one in a canal(next to swamps and a plain).
They are pretty fixed mechanics if you know how to do it. Alternativly you can use the paddle(first speed) to lose some time before going to anywhere.
You could also just see their "normal" spawns and mark them on your map or assume their next spawn. They actually make a splash so you can see them from farther away.

https://www.youtube.com/watch?v=lM3n8ahNwq0&t=1572s