After 15 days travelling around Thailand I could write lots of entries, but I have decided to start straight away with a tip. If you are around Krabi area and you are thinking in visiting the surrounding islands do it in a traditional longboat private tour.
I said private tour because they offer lots of group tours. Those ones, are 10 or 15 people in a single boat plus there are many other tours with the same timetables visiting the same places. With that, you get to be in a same beach or bay with lots of other people and you can't decide how long are you going to be there or when to change to another place. The experience overall is not as pleasant as you were expecting. The prices for a group tour are around 700THB ( 18 euros) per person. In my case, we were 4 people, and we paid 3200THB (around 68 euros) for the four of us, so we didn't pay much more each than in a group tour.
In a private tour you do your own timetable. If you don't like one beach you will go to another one, if there are not fish in this bay (sharks in my case) you will move to another one, if this side of the island is very windy you will sail to the opposite one... and so you enjoy much more your trip.
If you travel by yourself or as a couple you can always find 2 o 3 more people in the hotel you are staying, in the restaurant or just walking in the street. Anyway, my advice will be always to take a private tour, enjoy the experience and get the best shots without cuing :P