It could be tons of things. If its in a the marina, it could be because tons of people are using it. Also, one thing to check for is if you have a separate software for your wireless adaptor. This was killing my internet speeds when I first got my computer. I would have the Linksys Wireless Router Wizzard running to connect my computer to the internet, but the Windows application was running as well. These two programs conflict eachother, and cause it to be super super slow
Are you using a laptop or a desktop? It would be intersting to see what you get if you moved your computer to a different network (raise your sails and putter over to the nearest starbucks!)