A city of 250,000 people could have 250 boats (that's enough for a marina or two) and it would be 0.01% of the population (the one percent of the one percent). That seems to not really be that crazy.

And if you consider that a small percentage of the boat population may have 2 or even 3 boats, than it gets even less weird.

I also think that if you live near water, people are generally at least a little more likely to get a boat instead of a nice car or bigger house or other luxury item.

Edit: I was off by an order of magnitude so it would be 0.1% not 0.01, however, I think the broader point is still valid.