You can call for timings as the boat leaves at different times depending on the amount of people. I called 2 days in advance, the boat left at 10 a.m. and came back at 4 p.m. which wasn't enough time to enjoy the trip. If I made my reservation a week ahead then I could get a full day there. It's 16 KD for adults, 12 KD for kids 3-8 and 9 KD for children under 3. It includes transportation on the island to the restaurant and buffet.
You can call for more details:
Marina reservations: 22244988 or reception 90014123, the reception guy is really friendly and gave me great info.