We have been to Cozumel and Playa twice, both times last week of Oct. to first week of November. The first time 2008 was a tad cool. The second time 2010 we had to stay in pools or ocean almost constantly so I'm really thinking it depends on the year. We're going again this fall (again last week of Oct. to first week Nov.) and hoping for great weather.
In 2010 we caught a bit of a hurricane (outer bands) it was the day we went from Cozumel to Playa (we always split out 10 days over two resorts. The swells in the ocean were huge, it stormed on night and was rainy the following morning but burned off by noon, that was it.
I know those destinations aren't exactly the same as Isla but probably similar.