Fish will bite 3 times faster than usual, and loot will be much better than regular fishing.
The chances of loot are
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10% — some enchant book (Protection IV / Unbreaking III still the rarest within that slice, Protection II / Unbreaking II a bit more common, everything else as filler)
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1% — diamond
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4% — gold ingot
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~85% — untouched, normal vanilla catch (junk, fish, or whatever vanilla treasure roll already gives — nautilus shell included)