Float tube questions

This past four days, my wife and I went fishing. She had a float tube, a
Caddis, and all the stuff. I bought a new set of neoprene chest high waders
with the soft feet. I used her tube and flippers.

All seemed to go well, but she said I sat too low in the water. The water
got in over the top of the waders a little. It seemed like I would slink
down in the seat, and have to pull myself up from time to time. This is
when I got the water in.

I weigh about 190, and she about 120.

Question: How do I select my own tube so that it has enough flotation for
me? Are they rated by the poundage of the user? And do I need the round
donut type, the U-shape, or the two pontoon boat thinguses? I like quality
goods, but don't want to have to sacrifice any limbs for the purchase.

Some pointers in the right direction would be appreciated.

TIA

Steve