My choice would be aluminum if it's within budget. Pressure treated is likely cheaper, but I highly doubt it would last nearly as long in the same kind of environment. As for vinyl/plastic, I'd make sure that it's weather proof or more specifically heat-proof...don't want it warping in the hot summer months.

Have you checked out "dock-in-a-box" ?

Half of communication is listening. You can't listen with your mouth.