Is it possible to cast pre soldered neopixel rings into resin to make a waterproof light?
Yes that should be fine as long as you wire them up first. The only issue i can think of is that it may make trap the heat in if you are planning to run them very bright.
If i then place the units underwater will the heat dissipate through the resin enough?
I think it should be enough, I do not think each unit generates a whole lot of heat and the waterproof strips that you can buy are pretty much these units coated in resin.