I have had quite a few leaks in my float tubes over the years. They have always been very slow leaks that never put me in any kind of danger.

But... I would be very uncomfortable in a float tube where the insert (truck tire tube or bladder) was not incased in some kind of fabric or canvas or whatever they use and this is why... when I lived back East I used to float fish down warm water rivers using an inner tube like you described. I would just sit/lay on top and fish while I floated downstream (tons of fun if you've never done it!). One time I caught a nice largemouth and somehow it flopped so that one of the dorsal spines caught the inner tube and it exploded - literally! My sister was with me in another tube and she said one second I was there, then there was a big POP! and the next second I was gone.

If that converter thing is what I think it is (I saw one in a catalog once) then most of the tube is exposed which I would be very leary about.

Now when I float down the river back East (on vacation) I use an old float tube.