Diving for sea creatures is probably the quickest way to earn bells, but you can also sell fruit, sell crops that you can buy from Leif on Harv’s island, catch bugs and fish, sell sea shells by the sea shore, find your money tree every day and bury 10k bells in it’s spot(that’ll net you 30k bells daily if you keep up with it), and selling fossils. If you need any clarification on what I listed just ask! Always happy to help.