If it's done Creole or Cajun style, the crayfish are going to be pretty strongly flavored. They tend to have a pretty strong flavor all on their own anyhow.

So you could go one of two directions: something very pale and sharp-flavored, to give contrast and to give a little relief to the tastebuds.

Or you could go with something very dark and strongly-flavored, like a porter or stout, which would have a strong enough flavor to stand up to it. Porter and stouts also tend to have a little more sugar content than lighter ales and pilsners, so that would also help offset the flavors.

Something regionally-made is always best, I think. Lazy Magnolia's Reb Ale or Southern Gold Ale would both make good choices for a lighter-flavored contrast. I'm not sure their Jefferson Stout would be a good choice for a dark beer--with sweet potatoes in the brew, it might be a tad sweet, depending on how the crayfish are fixed (etouffe? or how?). A plain Sam Adams could be just fine.

Something lighter (but not too light) might be nice, especially if the weather is warmer outside. I would recommend a brown ale. Moose Drool is my favorite, but it's hard to find outside of the Pacific Northwest so you might have to go with something else. I'd say just go somewhere with a good micro-brew selection and pick out a brown ale. Like tracebooks said, pick a local brew.

A beer that is at it's best served very good-- and one that's flavorful enough to stand up to the seasonings. I would think Sam Adams Summer Ale would be great.