The weather will always be the weather... you hae no control! That being said... I have been working here for years and have only had a handful of weddings that were rained out. It is uncommon for it to rain endlessly and will most likely be a flash shower followed by more blue skies!
Here is the weather... as I see it. 95% of the time it is fantastic here.
Playa Del Carmen weather, In our own words:
DECEMBER, JANURAY, FEBUARY
This is the best time of the year to travel to the Mayan Riviera if you don't like excruciating heat. December and January, temperatures cool down and on cloudless days swimming is just beautiful. At this time of the year you get the least amount of daylight, but Playa Del Carmen & Cancun light up at night so it's not really an issue.
Humidity and rain are lowest during this period. If rain or a cloudy days do occur, things can cool down considerably. It is advisable to take a light jacket with you in order to stay comfortable.
MARCH, APRIL, MAY
Things start heating up mid-February and by April you can be roasting on the beautiful beaches of the Mayan Riviera. Water temperatures have not caught up but outside temps have started to rise.
The "rains" start in April peaking in July-August. This is a wonderful time to travel because after "Spring Break", things calm down until the summer travel season starts in late June. Water temps are warm enough to swim comfortably; it's hot but not "July-August" hot. Weather-wise, it's a great time of the year to go.
JUNE, JULY, AUGUST
This is the hottest time of the year in Playa Del Carmen, especially mid-July and August. During these months the Mayan Riviera beaches fill with vacationers!
Summer is also the rainy season however if it does rain, it tends to hit only once a day and for a very short period. Quite often you are happy it rained as it tends to cool things down a little, at least for a short while.
On a cloudless day in August the heat can be unbearable for some people. Shoes are needed because the sand can get too hot to walk on. Even walking any great distance can become a chore. Dehydration is a major factor during the summer months as well as sun-burn. At 12:00 noon you can get burned in 15 minutes without proper sun-block.
SEPTEMBER, OCTOBER, NOVEMBER
In September and October the ocean water is hot from the summer heat. The outside temperature drops making daytime temps hot but bearable. Evenings are beautiful and you can still swim at night and be comfortable. This is also hurricane season along the Atlantic coast. Hurricanes or tropical storms can occur anytime between June and December, however the common period is September - November. It is advisable to pay close attention to weather reports during these months.