Re: Tube going under...
I've gone through about 6 or 7 tubes over the years. My lastest one is the only one that does what is being described. It is also the "flattest" tube, I think they call it a deck tube.
I've noticed more and more modern tubes are designed flatter and closer to the water. Generally this is good, as the lower center of gravity makes them easier to get on from the water, and more stable when underway. But it also means they hold less air and have less buoyancy, and act more like a nose-diving wedge when they hit the water just right.
It's a trade-off. Generally the fatter tubes don't have this problem, but you need to be a bit of an athlete to be able to climb back on 'em in the water.