If you mean Sunset Beach resort I do not think they book online. We just took a taxi $2.00 pp each way. It is probably no more than a mile. You will see the Conquest in port from this beach. When we went the cost for a day pass was $20.00 for adults and $10.00 for children. I understand it has doubled since they put in a water park. When you get to the gate you just tell them you are on the Conquest and would like to spend the day at Sunset beach. They will collect the money and ask for your sign and sail card which you will ask for back at the gate when you are ready to leave.
We had a blast! For was great but if you want to eat somewhere other than the snack bar you must have appropriate clothing which we did not. Came in our swimsuits. I beleive you must have a shirt on with a collar. Cannot remember if shorts were permitted or sandels. You could call them and ask. The food at the snack bar was great but the open air restaurant looked really nice.
Hint" Bring a cup 20 or 32 oz with you as they only serve drinks in 6 or 8 oz cups. They have self serve soda fountains right there on the beach. By the time you get back to your chair it is time to go get another drink. They will even fill them with alchoholic drinks. For $40.00 it is all the food and drinks you want, water slides, non motorized water crafts, beach chairs and of course use of the beach. They do not accept tips at Sunset Beach. They have a swimup bar that is really nice! I enjoyed the pools.