We went thru in April on our way further South, no problems at the border but they sometimes try to charge you for bringing certain things in like for instance gasoline in containers. They also like to see documentation on boats, not sure if it would apply to kayaks. Have not been to the reef in a few years but have good memories of it. In April the road between Rocky Point and Mayan Palace was horrible, just a bunch of holes with a few shreds of pavement.
The border crossing coming back into the US will likely be a lot longer wait than on the way in. Do an early morning weekday if possible.
In 30 years never been stopped. A friend told me if they stop you and demand you pay an unreasonable fine tell them you want to pay at the station. Also keep two wallets, one with just a small amount in it for "show". Just what I've heard to do but myself never had a problem and been there dozens of times. Don't speed, but stay with normal traffic, not the morons who think it's OK to drive 90.
Brian, 2011 Winnebago Via Class A on Sprinter Chassis