*grin* I don't get the question much anymore, as being a meteorologist is not a family friendly career...strange hours, no consistancy, I'd equate it to being a nurse, with rotating shifts where I worked and so on. I'm a webmaster now, but you're right, I do get the questions from time to time and like to flex that "weather muscle" when I can (including talking at the kids' schools). Sooo, with that in mind, here's what I found for the Western in July.
Key West:
http://www.srh.noaa.gov/eyw/HTML/climate/eywclimate.html
You're looking at average highs in the mid to upper 80s, average lows in the 70s, and rainfall about 15 days of the month, but it seems that most rainfall is light on average.
Grand Cayman
http://www.gov.ky/servlet/page?_pageid=2353&_dad=portal30&_schema=PORTAL30&_mode=3
Pretty much the same, but with more chance of rain than Key West according to the stats on that site. October is the worst in terms of chance of rain.
Cozumel
http://www.weather.com/outlook/travel/climatology/monthly/MXQR0074
You're looking good here too, although keep in mind that the dew point temp. is not noted (the dew point is what is important in terms of what it'll FEEL like. Dew points over 70 tend to make it VERY uncomfortable for people...) so while it says ave. high of 87, a high dew point, which means a humid day, can make it feel hotter. But isn't that why we go to these places?
Castaway Cay does not have an official weather station on it, therefore no official weather records. Freeport, on Grand Bahama Island, has one...
http://www.grand-bahama.com/climate.htm That's pretty close to what you'll find.
In any event, I don't think cold will be an issue in July...enjoy your trip!!!
