In the Pacific Northwest try a company called Broadband Xpress. They supply WiFi exclusively to marinas. Not cheap, but they specialize in hi powered equipment. Google them.
I got their service when I was living at a marina. The card I bought from them was advertised as being 10x as powerful as regular WiFi. Again, their equipment is not cheap - start at C$100 and go from there. But if you cruise their site it may give you some hints or ideas
S/V Gandalf III