I'd
be wary in the Caribbean where Malaria or Zika are a possibility. Especially if you plan to have another one because either of you can become carriers and infect the other.
All Countries and Territiories
Zika Virus in US Virgin Islands - Alert - Level 2, Practice Enhanced Precautions - Travel Health Notices | Travelers' Health | CDC seems to rule out St. Croix.
And you don't really want to expose the current kid to Malaria drugs either right? Rules out a portion of Central America and the Bay Islands.
Hawaii is one connection away. Maui is 10-11 hrs out/back. Kauai 11/9, Kona is also 11/9 but depending on how much you spend per flight it can either be a mid-afternoon or an evening arrival. Mostly thru LAX which would give you chance to get off the plane, breastfeed etc.
If you want to try Honolulu (Oahu), United has one non-stop each way. 8h/7h. Many of the shops pick up in Waikiki if you don't mind crowds on the beach.Probably elsewhere also - I just don't know where. Since the boats are mostly at marinas not in Waikiki - possibly you could find a resort along the route?
Many of the Hawaii flights are going to be overnight returns due to the timezone changes. Since Zika started there have been 16 cases in Hawaii period.
The other thing to consider is that the south Maui operators all leave from the Kihei boat ramp and dive Molokini - about a 15-20min. ride. So you could be back as early as 10:30AM or so if willing to be there at 6am. Additionally in some areas of Wailea, there are resorts or condo complexes which have dive sites off their beach - even shops on-site. And dive guides are available thru them or privately.
West Maui can be similar - Tiny Bubbles is at the old Embassy Suites. And there's several condo or resort properties farther north in Napili Bay or Kapalua where the dive is either off your beach or a 5 min. drive.
The Sheraton in Ka'anapali is famous for being built around the Black Rock dive site - even named for it.
my .02