Hi Dave and Welcome
1) ANSWER 1. Do it how you want to do it. Take pictures!
2) ANSWER 2. We haven't seen one of those FINISHED. I've seen folks start renovations like that and have NEVER seen them completed. It would be really hard to get things lined up as it went back together and get fair curves. ANd one would need to be pretty handy with fiberglass. Seems that it would work in theory, but in practice? And please let me know when you find that supplier of affordable XPS foam blocks.
Now why not put on a marine grade plywood deck!
I've thought that XPS home insulation sheets could be glued or taped with silver HVAC tape and used as a substitute. Best option is to dry out the white structural XPS blocks and reseat them with marine grade flotation foam.
Cheers
Clark and Skipper
Signal Charlie