30 - 50 Fish in shallow water. Stand on the shore of islands such as Moonglow or Haven and walk around.
50 - 75 Fish off of a dock, a few tiles from the shore. As your skill rises, fish a little bit further and further out. A Crystal Ball of Knowledge comes in very handy to help gauge the best distance from where to fish.
75 - 100 Fish in deep water (gains after 85.0 are only possible in in deep water). Deep water requires the use of a boat to reach. Get on one and sail around. Be prepared to fight Sea Serpents.
80 - 100 Alternatively, as the method of DJ, after you hit 80, you can use SOS's and a Fishing Scroll of Alacrity. When you sail to the SOS coordinates, and use the alacrity scroll before fishing for the SOS, you'll get decent gains by pulling up the random loot before the chest. If you keep runebooks with close coordinates (NE, SE, NW, SW high numbers, and low numbers) and can dry dock your boat quickly and recall to a closer coordinate, you can get a few SOS's in before your alacrity scroll ends. Note that after around 95, it's slow and repetitive.
If you have a Crystal Ball of Knowledge, turn it on and try out different fishing locations until the ball reports "Optimal" difficulty. It will let you know exactly where the optimal spots are. The distance is about one step further away from land every 5 skill points. Once you reach 95.1, it will only ever say "Easy" even in deep water. Expect to catch around 3,800 fish to go from 50 to 100 skill(as well as plenty of nets, treasure maps, white pearls, delicate scales and footwear) while using the crystal ball under optimal conditions. If you just go straight to deep water after 85 without using a crystal ball, you may unknowingly be fishing in slightly over-challenging areas, slowing the rate of skillgain.