I have a Tamrac Explorer, the largest size for carry on. I have found it to be pretty good in moderate rain during a day out at motor races. No idea what sort of weather you might encounter in Costa Rica in July. I have an OpTech rain sleeve for my 7D though I have yet to use it.
Watch out for restrictions to carry on, bottles of cleaner fluid etc, also do not check camera gear for the hold, have you seen the baggage handlers out of the departure lounge window? I have!
Hopefully some one with experience of Costa Rica will chime in with something more usefull, also what sort of things do you like to photograph?