Doc Intrepid was nice enough to share his knowledge/experience of the following phenomenom with me:
Certain geologic formations, generally involving two or more islands in relative proximity, can lead to a condition known as a downwelling. While there are other downwellings as well, these result from tidal changes, and often channel water receding from shallow coral reefs surrounding islands through the constriction between islands and thence over the top of the wall where the coral drops off.
A diver caught in a downwelling is at a disadvantage, because they are difficult to avoid - swimming away from the wall can help mitigate the effect somewhat, but a great deal of water is moving...
I am aware of at least one fatality within the last 15 months that was reported to be attributed to a downwelling, the victim was an older woman diver who was swept from her buddy and down over the edge of the dropoff.
For many reasons, esp. with an 18# BC/wing, you would be well advised to consider carrying a device like a safety sausage while boat diving in the islands. Something at least 5' long should provide an extra 40 lbs of lift should anything go awry (even recovering a lost weightbelt can be difficult without some form of lift which is NOT your own BC/wing). Plus, it can be used to signal the boat should you be swept some distance from it during your dive. Good ones are made by Carter Liftbag, OMS, and Halcyon. They roll up quite small, and are not devices strictly for technical divers. IMHO, recreational divers diving off boats in the islands should carry them, together with a small spool you can use to deploy them as well.
I hope you don't mind me posting part of your message Doc! This what makes these boards!
I always knew it was appropriate to carry a safety sausage, but was not aware of this effect. So, I now have the 18# wing in hand. It is very compact and looks like a decent piece, but I won't be going diving w/o supplemental lift.
Thanks again to all for the input!
Greg in CT.