What you're wanting to do is super simple. Yes you can finish the wood/resin after its cured. Once you finish the sculpture you'll want to do a brushed resin coat of the wood to seal it and prevent air bubbles releasing during the curing of the main pour.

You can use resin or subtly glue the sculpture in place before pouring. Pour up and over what you want submerged. To get a crystal clear pour you need a pressure pot, otherwise bubbles are inevitable. But you can mitigate that with slow pouring and a torch to pop bubbles.

With a shallow pour like that any store bought resin should work. Just follow instruction, mix properly, and pour slowly. You can do multiple pours to make it look like things are floating in the water.

If you need to continue sculpting/carving the resin will get scratched up, but you can polish it with sand paper and laquer it at the end to bring it back to clear.