Zombies can't swim, they don't have the co-ordination to do so. However, as they don't need to breath, they can float [depending on their gas/water swallowed or inhaled ratio] or walk along the bottom of the sea floor.
In contemplating the boat scenario, the major issues would be if zombies were caught in stray nets or floating around ocean liners and cargo vessels. You could be hanging your line and attracting the zombies. It'd probably be easy to knock them off your boat, but depending on the size of both the boat and the amount of zombies, they coudl capsize it or clamber aboard whilst you were otherwise occupied.
Subsquently, the big issue is what has caused the zombies? Has it infected other animal life? Would the zombies attack other animals uninfected if they could not find humans to eat? So could swarms of zombies take to the oceans in an attempt to grab fish? However a pathetic attempt that woudl be.
Of course, things tend to degrade faster in the ocean, so the zombies in the seas would rot quicker and be less of a threat.
Wow it really is possible to talk about Zombie Apocalypse scenarios for the entireity of a first date.
And also, zombie narwhals sound terrifying (or a great name for a band) :)