It's important to differentiate between floatation tank and sensory deprivation tank. The former is simply a tank filled with water and epsom salts at body temperature. In the latter great attention is also given in order to cut off any other sensory input, mainly sound and smell. Most floatation tanks you can find that offer their services will not be that silent for example.
I tried an hour in the floatation tank, but the experience wasn't that relaxing as I expected. It is very difficult to be perfectly still while floating, so you'll probably end up touching the internal walls constantly with your head, feet or hands. Probably it won't happen to commonly if you're not too tall or if the tank is reasonably large. Disrputions like that are the main obstacle to a perfect enjoyment of the experience, and possibly the development of hallucinatory phenomena.
However, I think the main obstacle is time. One hour only is not enough in my opinion to fully appreciate the experience, and experience hallucinations. The same John Lilly, the inventor of the sensory deprivation tank, believed that it takes about 40 minutes to get used to the tank, learn how to float properly and start enjoying it. If I remember hallucination occurs after 2-4 hours. I would suggest you to read John Lilly - The scientist, his autobiography. It's really interesting, especially all the second half, and he describes accurately his experiences in the tank and those of other people.
For a while me and my flatmates wanted to build our own sensory deprivation tank, apparently is not that impossible. Pretty expensive maybe, especially the epsom salt tho