Many of the insects you find in the pond are actually larval (and sometimes pupal) stages of terrestrial ones. Here are a few examples:
Dragonflies have a larval stage called a nymph which are actually really amazing hunters.
Mosquitoes have larval and pupal stages that are aquatic.
Caddisflies also have three stages. The larvae make their own houses. Some make them out of stones, others twigs, and some even leaves.
Some insects are aquatic as adults like this water boatman:
Some aquatic organisms look like insects but are actually not…like this amphipod:
Amphipods are small crustaceans (related to crabs). Their common name is sideswimmer.