Have you considered downsizing your home to something that'll run you about $1800. per month (ie what you're expecting to pay for running costs on the boat). You could theoretically then save $1200. per month. After a year or so, you could buy a nice weekend cruiser all cash. Once that's done you could simply pound down the mortgage with your excess cash.
That'll get you on the water
, meet your saving objectives, and allow you to build some equity.