Terrible plan. Biggest part of which is that I forget what percentage of the population lives along the coastline, but it's MOST of it. So you'd be heading into MASSIVE zombie herds to find this boat. And you want a big yacht? Now we're talking bigger populations to find communities that can afford them. And they're huge gas-guzzlers. Fine if you just want a nice boat to drift off and die at sea in, but not much else. Every time you pull over for gas (and it would be OFTEN), you're heading into heavily-populated areas to find marinas to refuel at.
Correct answer (if you already lived close to water, not worth it if you're too far away) was SAILBOAT. They run on magic instead of diesel. One with a windmill/turbine on it (and many do have them) would even keep your batteries charged, so you'd get lights, electronics, fridge/cooking, maybe even desalinization if you stole a nice one. Harder to pilot, but a much nicer option.
Either way, key to surviving these sorts of things is to be as far from a population center as you can. preferably when it hits, but definitely head there after. Heading back into populated areas would be suicide. Long after initial outbreaks, it may be worth going back, if only to loot from the dead cities, but not while it's still this active.