Hi Melanie,
I am not sure if this would be something you are interested in or not, but Natasha (ScubaBoard member) organizes diving trips to various places all over the world. She usually tries to put together groups. You might try emailing her and finding out the details of how her trips work. She has posted a couple of trips reports here on the board, I think they were here in the Travel section.
If you would like to contact her directly, just click on the members link at the top of the page and find her name (she's on page 15 of the members list). Then just click on the mail icon next to her name and send her a message.
I think she might be able to help you out.
Cheers,