I would make a reservation, also, many of the big dive operations offer free lodging with a course...as far as meeting others, all the places are pretty fun gatherings of folks, and its a very compact town, one main road and 30 places to sleep/eat/dive.
I used UDC, and stayed at their super nice hotel the Mango Inn, up the road above town 5 minutes walk, swimming pool and nice cafe, good scene, both dorm rooms, private rooms and cabinas. Its more peaceful away from the main strip lots of motos and people in the bars till later in evening. On the other end of town, Altons and the Lighthouse looked very nice...more peaceful at that end too..