Hi, Tina! You will be asked to sign in for the Pirates and Pals Voyage about 45 minutes before you actually set sail, so you can enjoy the meet and greet and the snack buffet. You will be given an exact time of arrival when you call to make your reservation, and that time is based on the fireworks schedule for the evening of your cruise.
I would say to be on the safe side, once you are given your arrival time for the Pirates and Pals Voyage, plan to have dinner at Chef Mickey's about two hours before. That gives you plenty of time to enjoy your meal, have fun with the characters, take some pictures, and still make a potty stop before heading up to meet the party!
You can check out the park calendar at
www.disneyworld.com under the planning tab to give you a time for the fireworks the evening you are making your plans. Expect to meet for your Pirates and Pals Voyage an hour and a half or so before the fireworks are scheduled to go off.