I know this is way late but, You need to goto the beach where you make SOS with the rocks. Then goto the next area where you dive for items in the ocean. There is a flat rock on the right of the screen, swim to it and it should say "Start fishing". After you start fishing you will need to look at the fish in the pond, they all like different colors of bugs, but there are some that don't like any bait at all. Put the hook into the water and slowly move it near a fish, if a colored bug comes up near that fish then you now know what it takes to catch it. A black heart will appear if the fish likes what it sees. Place the hook in front of the fish's mouth and it should bite. If you get a fish on, you will slowly drag the fish to the closest net in one of the four corners of the screen, and then swiftly swing the fish into the net.
It is very easy to scare the fish away so be quick but go slow lol. HTH