I stay on Besar. Besar is mellow, Kecil is more for the party crowd.
Monitor lizards are everywhere, just take a jungle hike. I once stayed at Cozy Chalet in one of their seaview stilt bungalows and observed a giant one from my balcony digging a nest under my bungalow for a few days.
If you go diving you will see turtles. Otherwise agree a fee with a local to take you out in his small boat. If you say you want to see a turtle, he'll keep going until you do.
I had to book last month to get the room I wanted. Places like Abduls and Mamas seem to be always well booked in advance and are popular. Tuna Bay also always gets good reviews. IMO the West coast beaches on Besar are the best places to stay. You can access lots of beaches basing yourself there by either walking along the sand or through the jungles. You also have a good choice of places to eat and drink and dive schools. I would advise against picking accomodation that is far removed from everything and isolated.
The snorkelling tours are very good and worth the money although if you are reasonably fit you can access most of the good spots on foot and by swimming.