It's not toxic. Minor contact irritant at worst and is commonly used for sealing wounds and, in a slightly less irritating formulation in surgeries.
Holds just fine when wet. Water makes it set quicker, once it's set it is not effected by water.
Are cheap FW fish worth more and more delicate than $100+ each marine fish and corals? Superglue is used all the time in attaching corals to rocks in marine tanks.