Hey Dave,
I did this. Pick up at hotel (ex-pat, who arranged the trip) early, 7am IRC, and an hour or so to Chalong. Full day on the boat inc dinner (fried chicken and rice) and you keep as much fish as you want.
We took a couple of tuna back to the hotel, gave one to the receptionist and paid the cook 100baht to barbecue the other.
We did a cheap trip, 1500baht, there were only 3 of us on the boat and it was very basic. We caught a lot of tuna and one very exciting dorado. We stopped at some islands (which I should know the name of
) for dinner and a swim.
If I was doing it again, I would pay more and be careful about the boat and it's facilities (toilets etc) and maybe make enquiries about what dinner actually was. Ours was a very basic trip. That said, it was very enjoyable.