There are several other brands of tires available for trailers, but none as good or as well known as the Marathons. Goodyear kind of has the market cornered on this on. Carlisle is another brand, but all I ever heard about that brand is how they blew out a bunch. Bad reputation to say the least.
Marathon's are a 6 ply tire with a load rating C I believe which is more then adequate for a boat trailer.