Do you have a particular brand of the water based urethane that you like? I have an outdoor varnish project coming up and was planning on using my standard spar varnish but this sounds interesting. I tried using urethane many decade ago while working on tugs and ended up spending a summer scraping it off and resorted to oiling the wood instead. I haven't gone back to urethane again but you've got me interested.
Sea Kayaker Magazine