What are the insects young who live in the water?

There are lots of insects who have nymph stages to their growth pattern. probably the most dangerous is the mosquito which lays its eggs in pools of brackish water.
The eggs hatch into nymphs which in turn evolve into mosquitoes. the female mosquito can feed on human blood and in doing so pass on the malaria parasite into humans which kills hundreds of thousands of people every year.

On the other side of the coin is the most beautiful insect, the Dragon fly which also grows from a Nymph and is recognised by it's large wingspan and fantastical iridescent colours.