From the Super Mario Wiki
floating in the water.
A Tube Guy is a Shy Guy in Yoshi's Island DS that wears a red, green, orange, or purple innertube in water and constantly looks from one side to another frantically, then moves. The only difference between it and a regular Shy Guy is that Tube Guys can be on water, which makes it nearly impossible to eat and stay on land at the same time. If Yoshi gets into the water, he can eat it easily. They appear mainly during the boss battle with Bessie Bass, where they appeared as obstacles or ammunition for Yoshi if he runs out of eggs.