Circular polarizer useful?

Hi all. I am about to finally get an Ikelite housing for my Canon RebelXT. For now, I intend to primarily use my 17-40 mm f/4L lens. I was wondering about two things:

1. Is a circular polarizer at all useful underwater? I know I won't be able to rotate the polarizer once it is in the housing.

2. Will the Canon 17-40 lens + circ polarizer even fit in the lens port? I don't know how much extra room there is in there between the lens surface and the dome/housing.

Thanks in advance.

water is a really good filter. Too good sometimes and hence we use strobes, magic filter, etc.....

i would leave the polarizer in your bag for the topside pixs.

since you can't rotate the polarizer, you will lose all effect of the polarizer and still get penalized 1 f-stop for having the polarizer.