I've been invited on a pelagic trip in a couple of weeks. What lenses do folks like for pelagic trips on largish boats (fishing party boats) with a crop body? 300mm? 400mm? 100-400? 70-200? What's too long and too short in this scenario? Is zoom useful or required? Is IS useful or not on a rocking boat? This is the northeast in winter so light is not great. Also should I be worried about weather sealing on the camera/lens?
The 100-400mm will give you the most flexibility for birds changing distance to the boat. IS is good for light vibrations; it is of little use for strong rocking. Hand-holding the lens while rocking requires good motor skills to keep birds framed, and fast shutter speeds are required to capture the image. Super-teles are only useful on calm water, or if you are extremely good at hand-holding and tracking. A wide-angle to low-tele zoom (24-100 or 28-135) is useful for large birds or flocks coming close to the boat and people shots on the boat. Use a clear filter over the lens and a clear plastic bag or shower cap over the camera for protection from salt spray. Bring a towel to keep the gear dry and a lens cloth to wipe the viewfinder when fogged.
I think Stephen hit all the right points. For a trip in the northeast in winter, forget anything above 400mm. Actually, that's probably true of almost any trip on a fishing sized boat. If the boat is chumming along the way (which they should be), gulls and gannets should come in close. The faster the lens the better. The light can be great, but the days are short and you want to be able to shoot for as long as possible. And a zoom can be very handy for shooting birds in the chum at 10 yards vs. alcids flying by at 100.
I've only gotten my camera out on one winter trip, but I made a good choice by picking my lens that had the deepest hood. It was a bit misty and my glass stayed dry all day.
I agree with what's already been said. I have been on close to a hundred pelagics (mostly out of Hatteras). I currently use a 300 f2.8 with a 1.4x, but I also have pretty good sea legs and a lot of experience photographing birds on a boat. I would not recommend that lens for pelagics for someone without a lot of pelagic experience. Previously I used the 100-400 and think it's a good fit for all the reasons stated above.
From what I've learned, you want to use mode 2 for IS as opposed to mode 1, since mode 2 is apparently better for tracking moving objects. Also, I generally set the focus distance to the max on the lens, since that will help you acquire focus faster. Make sure you are using AI Servo mode instead of one-shot and I recommend using the center focus point in most situations. You also need to pay particular attention to your shutter speed; I would err on the side of a faster speed even if it gives you a slightly underexposed photo that you can fix in post processing.
You do need to be carefully about salt spray. I fried a 50D that way last spring.
I think the 100-400 is the way to go. I have used NikonD700 and 200-400 which was great and the zoom is very useful compared to a fixed focal length and adding/removing TC