I've been playing this game since December 2020 and have always had a thing for hunting down levi's, so here's what I can tell you about Leviathans:

- Leviathans ONLY spawn in the "Ocean Biome" and will never appear in waters labelled anything else

- Leviathans can spawn in large groups. the largest I've seen was 6 together, which I literally built a home on (google Living on a Leviathan. Part 2: The Quadruplets Reddit to see).

- Leviathans can spawn close to anywhere and it appears the only requisites for them are the ocean biome and a certain minimum depth (total height of storm wave + height of levi = minimum ocean depth they will appear). Aside from that, they can be very close to land or very far from it.

- Best chances/places to find a Levi are:
-- along ocean corridors (long strips of ocean between continents)
-- outsides of continents in oceans, not bays (if they spawn in a bay, it has to be huge and also have a sizable ocean biome)
-- a minimum of 3 or more continents/islands away from your spawn continent/island as this is usually when the oceans widen up or are at least more consistent in chains of oceans