It’s not really a set spot on the map, but not a lot of people know about Obolus nests.
Whenever you find an obolus idling underwater, remember where they were before you get it’s agro, because there’s an unmarked (no circle with bubbles above it) diving spot with 5 sealed chests directly under it! It doesn’t despawn, so if you remember where it is, you can come back every 5-10 minutes and they’ll regenerate!
I’m unsure if/how often the obolus respawns there, but usually you can just get another free 5 sealed chests without much effort whenever you come back.