As the subject comes up a lot, I went around all the travel agencies near the train station in Trang and took their numbers email addresses etc.
They are up here, together with some practical info about getting to/around Trang and the Islands
Don't they all sell the same things?
Everywhere I go there's a bunch of agents selling the same tours. There's usually only 2 or 3 companies that own everything. If a hotel has an agent just book there and ask for a discount.
Thais rarely respond to emails nor book things months in advance. Pretty much all tours and boat trips can be organised the night before you go. Thailand is good like that. You can walk around at 8pm the night before and line up a tour for the next day.
The main thing with Trang is after June some of the boats stop and tours finish. I was there in late June and they said the snorkelling tours ended at month end. So it's best to go from October to June.