water proofing brand new plywood non pressure treated deck is shed roof

We have built a 16'X16' deck with a storage shed underneath. We applied 4X8' sheets of tongue and grove, not pressure treated, spruce plywood on the deck surface as the deck is the roof of the shed. We need to seal the plywood especially where the plywood joints are to create a long-lasting waterproof surface. We live in Alberta Canada where the seasonal temps can range from -45 C in the winter with snow and ice to +30 C in the peak of summer. The decks exposure in summer is full sun AM until 2 PM then full shade until 9 AM again. We get an average of anywhere from 3 to 6 inches of rain during the summer. We checked out the "Deck Restore" product but website says will not penetrate new wood well. I have been trying to get information on a product made by AMES termed as "Premium Elastomeric Coatings and Paints", but can't get a reply from a dealer that supposedly carries it, which is ACE Hardware. The closest ACE hardware is 8 hours away, so we would have to order and have it shipped. Anyway curious as to what product would best serve our purpose to fully waterproof our deck floor/shed roof? Also would it be better to apply a paint/stainable all-weather calking to the wood joints before applying a waterproof finish? Thanks for any tips you can give us as we have been keeping the new wood surface clean and dry by covering it with a large plastic tarp during inclement weather and need to apply a finish as soon as possible before we put up a safety railing.