You will be arriving right at the end of Rainy season, so there is still a chance of Northers stopping diving on the North side. For that reason make sure you sign up with a reasonably big operation who have a boat on each side of the island. AKR, Native sons, Coconut Tree, Pura Vida are the a few of the larger operations on the north side who don't let themselves get caught out by bad weather.
These guys also use the most experienced DMs/Instructors in town, which as Warm Water says makes a HUGE difference to your dive experience. Although there are a few Gringo instructors in town with thousands more dives on Roatan's reef than some of the recently qualified Honduran DMs/Instructors. Many of these are also qualified Marine Biologists who are down here doing valuable conservation work.
If unlimited diving is a huge concern (by unlimited i mean you will want to do MORE than four dives a day) you will want to look at Coco View. The downside for some people is how isolated these resorts are. A taxi to the tourist centre of West End will cost about $25.
All the resorts I have named here are very active in the Roatan Marine park initiative which is doing an amazing job in Roatan. IF THE DIVE SHOP YOU CHOOSE DOES NOT REQUIRE YOU TO BUY A MARINE PARK TAG OR BRACELET THEY ARE ONE OF THE FEW OPERATORS NOT CO-OPEARATING, they really do not deserve your business, please make this part of your purchasing decision.
At that time of year you will find me in a 5mm full suit, but many people make do with a 3mm, it depends if you also have an insulating layer of blubber ;-)
If you are flying through San Pedro, expect your luggage to arrive a couple of days after you do! Any medications you may want, such as sudaphed bring with you. Make sure you have a lightweight rainjacket, make sure your currency is in denominations no larger than $20.
Enjoy
Will