More details from UO Guide

• 0 - 30: Buy from NPC Fisher
• 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.
• 100 - 120: You have to have High Seas to do the Professional Fisher Quest to get/use the 105-120 fishing scrolls. If your lucky enough to get a fishing scroll you will be on your way to legendary. You will spend long hours out to sea with slow gains the whole way. To pass the time do the fish monger quests (requires High Seas).

When a fishing spot has been "used up" a message will appear over the character's head "The fish don't seem to be biting here". This means no more fish, or skill gains, can be had at that spot and so it is time to move to another location at least 8 steps away.