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"Crystal Ball of Knowledge
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. "