Hello -- thought I should report back that we loved Ambergris and didn't have any problems with bugs. Just problems with leaving and feeling like I was going to cry as the plane took off.
I went armed with bottles of Skin So Soft Bug Guard, Off, Deet 90%, ready to buy hemp oil, etc. -- I might have applied Avon stuff twice but i don't think i was bitten once by a no-see-um and pretty much forgot about it. We stayed on the water, had an outdoor patio connected to sandy area & we were out there at dusk and in evenings, left our doors open so we could see water and let breeze in, etc.
It was freakin great. Ran into a few mosquitoes but the breeze was steady enough to keep everything away. We were on the lagoon side once and I thought I could see mosquitoes swarming so they might be more of an issue there where air/water is more stagnant.
This is easily our new favorite spot in the Caribbean. Thanks so much for everyone's responses.