yes, i respectfully would...i trusted the weather channel!
http://www.weather.com/weather/wxcli...graph/USFL0243
and they list 35 as the record low for key largo ever, not 18.
they also report 51 is the record low for April, when the OP will be there, a very important detail, so temps in the 40's is indeed very unlikely. it would be extraordinary!

i choose to mention key largo as it is the most northern key and i assumed it would have the lowest record low of the keys. Things typically get warmer the farther south you travel while in the northern hemisphere. Easter/grammer school spring break will be a big pop all in the keys, but i focused on collage spring break in key west due to posts about fear of drunks; and drunken rental scooter drivers are a serious issue and annoyance during that time period.

anyways, It would be extraordinary to have temps in the 40's in April! yes, a record breaking low could happen, but the point i am trying to make is the OP should plan for the much, much more likely much warmer weather. i just wanted to be helpful to the OP and correct some beta i felt was incorrect. my apologies if i stepped on anyone's toes! the weather here has me all cranky!i wish it was 41 right now, this "winter" so far has consisted of a few nights in the 60's and IIRC only one dip into the 50's. too dang hot