Thanks, all! This would be baby's fourth trip abroad in 10 months, so the travel part doesn't even phase us anymore, but our travel companions aren't quite so foolish.I will look into the Keys and see what we can find for shore diving there. (Leaving the girls with strangers is what most worries us about the whole idea, so shore diving would let the couples take turns watching the girls and diving, and no nursing pair would have to be apart for too long.) Thank you all so much for your thoughts and suggestions. I really appreciate it.
I have not been to the Keys yet myself but I am planning a trip soon with others from my family. Here is a thread that might be useful:
http://www.scubaboard.com/forums/florida-diving/451930-seamonkeys-water-sports.html