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The old-style "linear polarizer" (LP) filter causes problems with modern AF systems, that's why you want the "circular polarizer" (CP) type.
The main purpose for using a CP is to control reflected light. For example, it's great for reducing the amount of sunlight reflected off the sea surface, in a "seascape" photo; perfect for many of the photos you're likely to take. The CP will probably be very useful for whale photos, especially if the scene is somewhat "back lit", with the sun beside or behind the subject, as opposed to behind you.
Another reason for using a CP (which is also associated with reflected light), is to increase the apparent contrast in "skyscapes". I would not use a CP for birds in flight (BIF), but I probably would use it for many senic photos involving lots of sea and/or sky. For BIF with a sky background, I generally use at least +1 EV exposure compensation, to get sufficient detail in the bird.
Also, most CP filters reduce the amount of light getting into the camera by about "2 stops". This can help you to get slower shutter speeds for capturing "motion blur", when that's a technique that you want to use (classic example, foamy waterfalls). For example, if you're shooting on a fairly sunny day (EV = 15), the 'correct' exposure at ISO 100 and f/16 is 1/125 sec. This won't show much motion blur for 'moving water' (but it will for fast movers). If you slap on a 2-stop CP filter, the shutter speed drops to 1/30 sec., where you might want to be.
I'm a Canon user, but I can assure you that any Nikon drop-in CP filter for an AF lens will be a CP, not LP.
I use B+W Kaesemann (KSM) MRC CP filters. There are many threads on CP filters that you can find with the 'search' function. Just, try to stay away from the "should I use filters?" threads...