The only time we ever had a dive canceled was in 2005 in Cayman Brac when Hurricane Emily was coming. Then we only lost one dive and that was so there was enough time to get everything put up.
Over the years, I've seen a lot of weather on the Brac. In general, it tends to run in "streaks" - you can get luckly or unlucky.
A decade ago, the typical fall pattern was to have an intense thunderstorm like clockwork every night at around 2-3AM.
One year, we had rain during the day for ~7 days in a row. The guy who was with us on that trip was blamed as being a "Bad Weather Magnet", since it all cleared out as soon as he left and the next week was really nice.
Even with rains during the day and every day, it generally wan't that much of a problem - - the storms typically blow through in an hour or two...I've had trips where at the start of the dive it was overcast (but seemingly OK) when we dropped in the water for a dive, come back an hour later to find the sun out but the topside crew wet, cold and miserable...and we had no clue that it had rained while we were down.
Bottom line is to expect some rain; pack along a good jacket and use it when appropriate. If the weather's really bad at ~8:30AM when the diveboat's typically getting ready to depart, they'll simply just delay for an hour and let the storm blow over.
And with a departure this weekend, since there's no meaningful storm activity right in the Cayman region, your flights should be relativley uneventful.
Checking NOAA, there's a tropical wave that's approaching Nicaragua that's expected to make landfall by Friday...its northern edge is right at the Caymans. There's also a broad area of Low pressure around 200 miles ENE of Curacao, moving west at 20-25mph. There's currently a clear sky gap between the two systems, so I'd expect Sunday-Monday to be nice...its too soon to tell how far north this second system might get.
-hh