Last fall there was an accident involving the Cornish Vt VFD airboat operating on the Connecticut River. It took water over the bow and went down quickly. They were evacuating an injured person who was strapped to a 'board' and she drowned. A crane was required to lift it from the bottom. I have not yet seen a final report in the press yet and there may not be one until next year. It received coverage in the Union Leader for a few days.
This incident was discussed on another forum I visit. There was a link there to an article by "The Florida Airboat Trade Association," which implies these craft are deficient in floation. Here is the link to that thread...
http://www.thehulltruth.com/forums/t...ode=2#M1165079
The article I referenced is on what is now a dead link.
If you are planning to use this type of boat only over ice and shallow waters I'd expect you'd be fairly safe. Avoid the temptation to take it over large stretches of open water where the waves and wind can work against you unless you are sure the boat has sufficient flotation and stability for those conditions. I'm sure you can get more information on this type of boat on a Florida or Louisianna site.
Good luck!