Mosquitos like different environments than Sand Flies/No-see-ums
It's no surprise that Ronbeau and Victor (above) found no mosquitos on the ABC islands. They are "desert islands" by almost any definition. In Bonaire it is even more apparent, I liken it to a convection oven. Mosquitoes are unlikely, Scorpions? Maybe.
Mosquitos need still, fresh water (ponds, or even old tires) to breed. They do not like even light breezes. They need fresh water moisture and still air.
The best No-See-Ums anywhere in the Caribbean can be found on the Bay Islands. I suggest you steer clear, but truth be known, they are all over the Caribbean basin. They thrive near any wet decaying vegetable matter, loving inter-tidal wash zones (look out beach combers!), moist shaded gear locker rooms... even wolmanized pressure-treated wood (Sun deck loungers beware!). They attack predictably at dawn and then again in the late afternoon, say 4-6 pm or so.
Certain islands where you find heavy populations of one critter or the other, sometimes have "free zones" where nature's elements conspire against them. As bad as the Bay Islands are for sand flies, the South side is comparatively devoid of the critters, and rarely a mosquito. (Understand that "comparative" is a comparative word)
Argue as you will about the best goo to slather on your flesh to repel either pest, but I prefer (along with the US Military and recognized science) a liberal use of DEET. Contrary to urban myth, DEET has never been found to have a deleterious affect upon Corals, but why not shower it off before your dive, anyway?
Percentage of DEET concentrations available to the consumer vary greatly. The point? It all has to do with how much stays on your flesh while you sweat and go through a normal day. If you want to apply it less often, use the high percentage. If you are in and out of the water a lot, might as well go the cheap route and get the lower percentage version. There is really nothing more to it than that. Yes- most spray DEET formulas will melt plastic, so keep it away from your watch crystals and lens dome ports on u/w cameras. Yes, I have witnessed this disaster.
Once you are bitten, there are also another 1,278 remedies for the insane itching. What actually works is a q-tip daub of NASONEX (or the active ingredient mometasone furoate monohydrate in a suspension form, ask your physician for a scrip). It is a very powerful steroid and will make the itching go away instantly and stop for a very long time.
NASONEX (absolutely, no doubt) works instantly on no-see-um bites.