Why yes of course.
It is no secret that we are very active as a family. I believe that if the energizer bunny himself saw my kids even he would give up- my kids have extra tanks of energy (now all I need to do is bottle it).
This last weekend we went to the lake house, which, to the kids is the epitome of summer fun. There is a lake, a beach, a boat (with tubes and skies), a wave-runner, and tons of family to play with (not to mention the games and the food...). In kid world there is no downside.
I have never been tubing. But all of my kids have- even Pixie- at 19 mos.
We were on the boat trying to decide who's turn it was to go- Peas wanted to go and she had been so patient letting the other kids go first on the wave-runner so she went first. Pixie tried to climb out of the boat (which is really hard given the giant life jacket she had on). She was going to go as well, there was no two was about it.
When Peas came back on the boat. Pixie again resumed her pointing and yelling that it was her turn. We obliged. No she did not alone. We got her on the tube with her uncle and he proceeded to fall off the tube almost dropping her in the water. In the end we get her on the tube and she tubed. Holding on the the handles, grinning.
A 19 month old tubing? Why not? She will grow up just like B's cousins. Lots of water fun at a young age. Soon enough she will be skiing. I don't remember when B learned to water ski, but I know they will all learn before they are 10.