I've been doing quite a bit of reading (with high hopes) about the Panama Canal, and would appear that May or August would both be hot. Both are considered "summer" Below is copied from one of the websites I found. Absolutely facinating, I have two books on order from Amazon.
Robin
Best time of year to travel - October through April
Climate -
Average temperatures:
Winter months
Average temperature is 85 to 95
Summer months
Average temperature is 95 to 105
Average length of cruise - 10 - 11 nights
Carnival, Royal Caribbean, Princess, Celebrity and Cunard have ships sailing alternating routes from October through April, sailing through the Panama Canal. All cruise lines have repositioning cruises from the Caribbean to Alaska through the Panama Canal in the spring and the fall. These cruises will range from 10 to 21 days.
Robin
Best time of year to travel - October through April
Climate -
Average temperatures:
Winter months
Average temperature is 85 to 95
Summer months
Average temperature is 95 to 105
Average length of cruise - 10 - 11 nights
Carnival, Royal Caribbean, Princess, Celebrity and Cunard have ships sailing alternating routes from October through April, sailing through the Panama Canal. All cruise lines have repositioning cruises from the Caribbean to Alaska through the Panama Canal in the spring and the fall. These cruises will range from 10 to 21 days.