In reply to The Lemming:
I use a Marumi circular polarising filter on my 77mm dia. Sigma. There's no vignetting even at 18 mm but it is a very slim filter. The downside to that is that with the hood on the lens, I can't hold the edge of the filter to rotate it.
For the OP, rotating is useful (if you haven't already learned this) because you can vary the degree of polarisation between none & full by turning the filter through 90 degrees. Circular polarising filters cut down reflections and reduce glare without severely darkening the scene. The reflection thing is very obvious on water. The reduction of glare is not so obvious until you realise the entire blue sky is one big glare. Stripping out the random light deepens the colour dramatically. The filter does the same thing to everything else too: sometimes increasing colour saturation and contrast so far as to make a scene look rather artificial.
Something else to be aware of is that there's a strong effect at a right-angle to the sun and it tails off to practically nothing when your shadow is directly in front of or behind you. With a wide-angle lens, that can make part of the sky very deep blue, with a 'normal' pale blue patch on either side.
If you have a good lens with a very large diameter, and a camera on which you can push the sensitivity up a bit without compromising quality, the couple of exposure steps of darkening caused by the filter shouldn't give you any trouble. For photographing sailing events I'd expect no problems unless you want individual drops in the spray to be perfectly crisp. White hulls and white sails will look a bit like an advert for a tropical beach holiday with full filtering but rotate the other way and all the haze and all the sparkle will reappear.
If you're photographing small boats, Lasers of Fireballs or whatever people sail nowadays, I'd guess you'll be on the bank or shore and some distance from them so you'll probably be using a longer focal length, in which case vignetting won't be a problem. Camera shake and depth of field are more likely to ruin the pics so the slight darkening may be more of a problem. If that is the case, I'd suggest also buying a good robust tripod.
I keep my filter on all the time as 1) I rarely use the hood and 2) a scratched filter is cheaper to replace than a scratched lens. If you do buy one and you don't suffer vignetting, try fitting a cheap UV filter on the front. UV filters do nothing at all on DSLRs but it will protect the CP filter and will give you a bit more to grab when rotating. If it does affect the corners of the photo, you've only wasted a few quid.
I originally had a Hoya Pro on this lens but it broke while on the lens with the lens cap on and the camera in its case in my rucksack. It had lasted for over a year (no more warranty) and just went with no provocation. I suspect there was a small chip in the edge of the filter glass that was just waiting for an opportunity to propagate. Regardless, there's very little to tell between a good Hoya and a good Marumi - I have three other Hoya filters & just thought I'd give another brand a go.
D