Ridgid Plastic Sit-on-Tops:-
These have solid moulded plastic hollow hulls which sit low in the water. They are robust, stable and easy to kit up on. Looked after they will last for many years and are resistant to sharp bits of metalwork and diving equipment. They are the usual choice for the kayak diver.

Inflatables:-
There are a few which are suitable for diving. The advantages of inflatables are that they are incredibly stable and seaworthy, they have a huge load capacity and they are easy to transport and store. However, they can be more awkward to kit up in and they can get pushed about a bit by wind. There are some real cheap (crap) ones around which are a bit like lilos with sides but the proper ones are made from very tough fabrics and are often double skinned. Inflated they are surprisingly ridgid.