Thousands and thousand of species of insects...most of the flies, many beetles, neuropterans, caddisflies, etc...
But not salt AND fresh. Relatively few insects live in saltwater (especially not ocean). Mosquitoes are freshwater and while a few species tolerate brackish water, they really aren't saltwater critters.
I think it's a good bet to say that just about no wormlike critters, larval or otherwise, survive in both salt and freshwater.
Anyway, easiest way to identify your larvae is to let them mature and capture the adults. Adults are MUCH easier to ID!