Thanks for the constructive advice! I'm over an hour from the beach and just happened to be there for other reasons. I wish I had a surfing buddy that wouldn't be scared by my bulge! It would be interesting to surf fully pumped, if possible!
What that observation taught me is high tide varies considerably up and down the coastline, so its not as simple as locating a bouy along the same longitudinal lines as my home. I have more or less abandoned this theory, but anyone living along a coastline is welcome to continue the experiment. What I did find was that by following the tides, I was pumping more or less every 24 hours, and often again after only 6 because there are sometimes 2 peaks during that interval. Without exception, my bag was pliable and ready for another session by the time the next tide rolled in, so I think it was more about the timing and less about gravitational pull. besides it takes a lot of commitment to tube up at 3:00am on a weeknight.