Everywhere I have been in the Carribean asks you to fill in the PADI disclaimer form which asks a lot of health questions - including one on diabetes I think. I suspect answering yes to any of the questions will rule you out of diving. It's an insurance and more than me jobs worth guv sort of thing.
I'd phone ahead and ask the operator in Barbados. I have used
DivePro Barbados - Homepage in the past and thought that they were good.