Belly-boat / float tube = Typically these are innertube shaped or U-shaped. You sit in the water (about waist high) and propel yourself with fins.

Kick-boat / plontoon = These are 2 inflatable pontoons attached with a metal frame (think catamaran). The occupant sits in a seat on the frame above the water and can propel themselves by either putting their feet in the water and kicking (with fins) or by using oars 'attached' to the metal frame. Kickboats are much faster and have tons more carrying capacity and storage. They are heavier and more trouble to move around.

Hope this helps...