What craft were the captured British sailors on?

They were on a RHIB (Rigid Hull Inflatable Boat). Think of a Zodiac with a metal bottom.

They used the small boats because they were in shallow water and it keeps the frigate out of small arms, light rocket fire. The Frigate draws 20 ft of water and (I believe) their actions were hampered by the ROEs and the nature of the surprise.

It seems that the first group of Iranians approached in a friendly fashion and things got nasty when the other Iranian boats showed up. I guess that by the time the captain and crew of the Cornwall figured out what was going on, the 15 sailors/marines had already been whisked over the border.

the 15 british sailors and marines were not on the frigate HMS Cornwall at the time they were apprehended. They were on a small craft out in open water. Iranians didn't board the frigate they stopped the smaller craft and took the 15 prisoners

As has been said they weren't on their ship. They were a customs/inspection detachment, and were intercepted disembarking from an Indian vessel that had just cleared.

The Indian captain has also confirmed his position at the time as being in Iraqi waters, providing a reasonably independent source.

I believe they were on inflatable crafts.

those rubber crafts meant to go to shore on, you know? i think they were used as bait, plain and simple.