Carol, what month do you think is best to visit your freinds?
I would say the coolest month is the best month, cool but light winds, but we don't know that now do we? (Or we would make a mint predicting the weather). 
I'd say December is best (if it is cool?), but not around Xmas/New Year's ; then generally January seems to be the most reliable, but waters can be stirred up more with stronger cold fronts/winds. Earlier in the season is always better. February 09 with President's Day being early (don't go between Feb 12 and Feb 22nd 09 as it will be too too busy), & March with early Daylight Savings Time and Spring Breaks, Feb/March will both be tough months and too crowded a lot of the time.
Oh, water is 68-72 and air can be from 30 degrees to 60 degrees in the am - don't go if it is warmer than that in the am.
As said, manatees don't exactly make it easy for us......
Best, Carol