Some of those guys with expensive boats have them removed from the water, shrink wrapped and put on cradles. Other have some kind of bubblers or impellers to keep the water from freezing. I'm glad I never had a boat. They are an enormous, expensive burden. There's a subculture of people here that hang around in those marinas on their boats and wander back and forth from one boat or canal house or bar to the next...the whole culture is centered around drinking. I couldn't imagine living on one of those canals. The noise bounces off the water and those people party until dawn every night of the week all summer.
The general rule of thumb is 10% of the boat price is what it will cost you to run and maintain it. More in some places, less in others, and it depends on how much you actually use it. Booze and food is extra.