Hello Diverrex
Having been to Roatan 3 times so far and returning again on December 1st this year, each visit at a different time of year, here is what I can offer:
November is during the rainy season so odds are you will enjoy better visibility in May. The best vis I have experienced during my visits thus far was in April but you can never be sure, especially if there has been recent heavy rain or strong winds.
Sea Life is really good all of the time but to increase the chances of seeing a Whale Shark, you would probably have better odds in May than November (other SB members feel free to correct me if I am wrong as I have still not experienced such an encounter yet).
Water temp does not change much from my experience, stays around 80 degrees F +/- a few degrees.
Bugs - no-see-ums in particular - are the local biting pest. A good rinse with fresh water after being in the ocean and an application of diluted Avon Skin-So-Soft worked well for me in the past. Try to avoid being out on the sand at dusk as this seems to be the peak biting time.
As far as crowds, depending on which resort you stay at, that is the only "crowd" you will experience - unless you visit the area near the cruise ship dock on a day that a ship is in port. Most resorts are fairly isolated so it is just you, the resort staff, and fellow divers. The West End area of Roatan does have quite a few small hotels and dive operators so it has a few more people but nothing I would consider a crowd.
Regardless of when you go you should really enjoy yourself. If you have not dove along the Belize Reef before, it will be a real treat. Make sure you get the divemaster to get you to at least one dive site where you are highly likely to see Spotted Eagle Rays - one of my all time favorites in Roatan.
Happy Diving!!!
NW_DiverDude