Ahaaa - Whale Watching - one of the reasons why I picked up a dSLR and came to Pentax.....
The family and I took an Alaskan Inside Passage cruise and followed up with a whale watching trip on our last day up at the Kenai Fjords. I had a small canon point and shoot that I really liked - perfect size, but very sloooooooow on the shutter. Push the shutter and then wait 2 minutes with the Jeopardy theme song going through you mind and then it takes the picture. The result was that I have a 100 images of water and just one image of the whale breaching in front of the glacier.
Also, on the boat were 2 brothers who were National Geographic photographers, their wifes and their father. They had taken over one of the tables in the cabin with their camera equipment - like 6 bodies and you name a lens and they had it. The brothers were burning through 10 frames a second, the wifes were swapping bodies, batteries and lenses based on what the brothers wanted and the father was siting back and watching the whales. They easily took 5,000 frames that day, between the two of them. We saw a lot of whales too.
After that, I went shopping for a dSLR.
First - what crewl1 said - double....... So, take
fully charged batteries (as all the batteries you have). There were folks who burned through their single battery and then were done for the day - left to just watching the sights - which is not really bad, when you think about it. One hand for the camera and one hand for the boat (so as to not go swimming). Hard to really focus with all the bouncing around, even with two hands. Keep the camera strap around your neck - as it can be easy to loose the grip on the camera. Depending on the type of boat and the setup, you might want to take a small towel to use as a lens rest somewhere on the boat to help steady the shots. Continuous shutter, almost as a spray and pray. Get good focus (auto focus is the key here) and start shooting early and catch the entire action if the whale breeches - trying to catch the entire sequence. It is probably going to be bright out there with lots of glare - especially off the water. Take some lens cloths and lens cleaner, since you will get salt spray on the lens. Take several large SD cards. You can burn through 500+ frames very easily. Something like the DA 55-300 autofocus would be the lens to stick with. The 135 end might be a bit short, but it is all based on the boat driver and just how close the whales want to get.
Good luck.....