Hi, I have been through the Panama cannal on a cruise ship and it is very hot and sticky, it could well be like a tropical rain forest, hot humid and whilst there could be some rain or cloud you can also get very sun burnt.
The ship inside has air con, but you will want to experience the transition of the cannal.
The best way is in short bursts, at the main locks, three in all, and then occassionally check on the canal.
I found Acapulco, Mexico in May quite nice, more refreashing than Florida, but still the sun can have a big effect.
Hats and suncreame advisable, and also headache tablets if you suffer from direct sunlight. I do not usually suffer but had a migriane on the Panama night.
I found Cabo st lucus very hot and dry, and whilst the views are terrific, plan for a hot dry day.
Havent done Quatermala, but I went to Costa Rica, and there it was tropical rainforests, hot humid but liable to the occassional downpour. Beautifical wildlife!
All that said I have booked again, it is well worth it, but plan in advance.
Electrical suggestion, Camcorders and cameras, sometimes it is so hot or humid they can cut out if left in direct sunlight for a long time. I had one which had a 'dew point' cut out and if you moved quickly from Air con room to the outside it would 'cut out' for a period of time.
Take care on these and the film inside as some makes, do not come out afterwards.
On the Panama crossing day I found at Lunch, the inside resturants were very busy, I think everyone overdid it and wanted to cool off, in quality air con.
Final thought, anyone on medication, take care and consult your doctor, your tablet level might need adjustment, as you will be hot and may sweat a bit more.
Asthma sufferers be careful of LA in the summer!