Find out what is available on the boat before you pack. If there is a galley or snack bar, you may be able to avoid carrying snacks or beverages, making your pack lighter.
Avoid taking anything larger than a day pack. You are not likely to have much storage space and won't want to keep track of a large bag while trying to watch whales.
Bring things that will keep you warm and dry (unless you're whale watching in the tropics). Jackets, rain ponchos, warm shoes and socks, and even gloves may be appropriate, depending on where you are.
Wear as much as you can to avoid packing it. A jacket (light or heavy depending on where you are), poncho, hat, sunglasses, binoculars and even cameras can be worn or carried when you board the boat.
Take food items that will help you avoid seasickness, such as Saltine crackers or gingersnaps, even if you've taken a seasickness remedy prior to boarding. Plan to snack every 15 to 30 minutes to keep something in your stomach.
Make sure the necessary equipment is readily accessible. If your video camera is stuck at the bottom of a camera bag, you're likely to miss some great shots.
