I say get an !! shark, use a good map, pick 2 spots, activate gold rush, and sweep everything up. I recommend Mr. Snappy. He can swim between the 2 locations with ease and can eat in seconds as well as stun, If you can't get him, use the best shark you have. Equip power ups like gold magnet to increase gold earn.
I think to farm gold get a OP Sharkand use it to eat and kill stuff, following enemies are pretty good, Subs (Mini and Reg,) enemy sharks, the big ones, I don't know the rest but they are probably the best ones