I'm going on Holland America (Carnival-owned) to Half Moon Cay this fall, and the brochure that I received said that there were kayaks available for rent. They didn't stipulate how many or for what price...
I was at Half Moon Cay in 2002, and it sounds like they really "super-sized" since then... far more activities listed.
I'm assuming that Carnival and Holland America go to the same "Half Moon Cay"... but I could be wrong!!
Have a lot of fun... I've still got 60 days to wait before we go on the Westerdam!!