Hi Lavendour,
The hurricane season usually ends around November, so it's good time to travel. Actually all year round is good, as long as there's no current hurricane to prevent you from enjoying the beaches.
I do not know much about scuba diving, have no interest in it, but the beach is safe for all activities all yar round, no seaonal activities (as far as I know) Our winters are not cold for most foreigners, just cold for us locals who are mostly accostumed to a tropical climate. Winter would start around December back to February, roughly.
Hedonism Resort in Negril has a nude beach, not sure if it's Hedonism I or II, but I was there once and saw people naked on that side of the property (which I happened to wander off into without realizing) that was like 10 years ago though, late teens 
For backpacking accommodation I can't say much, I only know of 3 star hotels that may be suitable for a backpacker's budget. I'm not aware of any hostels. There are quite a few nice 3 star hotels on the beach in Negril or in West End, Negril. Negril is my favourite resort/beach spot
Try googling Negril beaches or check out expedia.com for rates.
Best of luck.
Samantha