Charles, gorgeous pictures as always! #2 shows amazing detail, especially the contours of the exoskeleton and the muscles.
The first picture looks to me like a larva of a hydrophilid beetle (Coleoptera: Hydrophilidae), but I could be mistaken. And you´re right about size varying with instar.
For further ID you could, (and that´s the way I myself would go)
a) ask a friendly specialist on water beetles who may, due to experience or access to literature, be able to say more even from looking at photos, if the photos show the crucial structures.
b) search for the literature, i. e. a published key on water beetle larvae that covers your region in order to figure it out under the scope yourself. This might be tricky as possibly such a key is hard to find because it´s older and not digitally referred to, or the one you find is outdated, or there is no key because yet no one has written one on these larvae (most keys on insects refer to structures of the adult forms), or...
... so, you see if you wanna figure it out yourself, you should go back to a) and ask the friendly specialist about such a key.
. See, parasitizing colleages is often the most parsimonious way.
No, just kiddin´. Honestly, I prefer reciprocal altruism.
(I can name North American specialists on (water) beetles who you could ask, if you want me to.)
The second picture: To be honest, I think most biologists won´t be able to say it´s a fresh water isopod (Crustacea: Isopoda) from this section only
. If a fresh water isopod was from my place in the Western Palaearctic region further ID wouldn´t be to difficult since there are not many species of fresh water isopods. Possibly, in your place in the Nearctic region fresh water isopod diversity is also low and scientific literature with a key should then allow further ID easily.
Hope this helps a little bit.
Cheers,
Betty
Amazing details. 
