I had the same concerns this past spring, I live in Arizona and I have seen snakes in the ponds and canals. I still went ahead and got a one man pontoon, I sit out of the water and the only part of my body that is in the water is my feet up half way to my knee. I know some tubes can also sit you high like that.
I was told that snakes don't come near me and so far it is true. I have a great time tubing around where bass boats cannot go or in small ponds. I say don't worry about the animals you mention, snakes are also in ground and fish don't come near you because you need to be re-adjusting you position every so often, (I use fins also) so you are moving your feet most of the time you are in the water. Enjoy yourself and get a big bass!!