Depends.  if its is just rain who cares.  If its storms the wind could
be bad that can make the channel significantly choppy and rough.  The
authorities may stop the diving.

It rains everyday (almost) at this time of the year - my experience has
been late afternoon or evening.

I have never missed a dive trip in Cozumel due to weather - not that it
doesn't happen - just hasn't happened to me.

I assume that you have never been to Cozumel.  You may get afternoon
showers but don't worry.  

Ron Lee

Being a tropical climate, rain showers are not uncommon... When I was there,
we ran into a few on our way to the dive sites, but they weren't any big
deal... A couple of minutes of getting wet and then you were through them
and drying out... Seemed that we were usually dried out right before it was
time to get in the water.. <grin>  If the winds aren't bad, I wouldn't
expect the boats to not go out... If the showers are localized and bad
enough at one site that it might compromise recovery of the divers, I would
suspect that they would choose another site...

Problem with getting wet?