Couple things I found helpful. While it is tempting to just cast as far as you can right away, it is better to clear the way of the closer fish first so you don't have to try to weave through them so much and lose points for a couple bits.
Bombs don't just work on sharks, turtles are nice to clear out too. Remember to go to menu and save once in a while... frustrating to do a bunch of work and have it lost b/c of something stupid.
And unless you want your game to glitch out don't power reel into the cave at the bottom.
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