fishin in the sea

[Fujibei]

When you go out to the ocean for fishing, do you say "go fishing in the ocean (sea)" or "go fishing on the ocean (sea)? You are on a boat or ship from which you fish, so you are not actually in the ocean (sea). That's where my confusion comes in.
The same question applies to "fishin in the lake" or "fishing on the lake."

[probus]

Always "in the sea" when you are fishing, rarely or never "on"

[Rover_KE]

I know people who say they are going sea-fishing.

Rover