Washington State and B.C. marinas get Wi-Fi: An entrepreneur has unwired 21 marinas. Service can be reached in some places 2 miles out of the marina, and costs $6.95 per day or $39.95 per month. Better yet, the company says they have 1,000 people signed up, although from the article it's hard to tell whether that means "sessions and subscriptions" or just subscriptions. $40,000 per month would be a nice starting point, if it's all monthly revenue.
Although another group, Wheat Wireless's TeleSea division, claimed to have unwired marinas around North America, their coverage map hasn't been updated since 2002 and their press releases end in March 2003. (The company posted a comment below -- has anyone used their service in any marina?)