Anyone know a polite way of saying I'm not interested? YES!! "I'm not interested."
Women are taught to be nice. That if a guy is a nice guy and isn't rude/aggressive to the point of near assault, it's mean to not want to go out with him. So the impulse is to, instead of actually saying no, assuming that anything short of enthusiastic agreement counts as a rejection. The problem with this is, taken to its logical end, you can end up marrying the first non outright jerk who is persistent enough if you never actually say no.
Guy are taught that women are to be pursued. So if a woman is giving reasonably positive responses, then the impulse is to assume the signs are good and he should continue.
You can see why problems arise when these two approaches collide.
In this particular case, you never actually said no. When he asked if he could walk with you, you said yes (encouragement). You chatted with him politely, let him sit next to you at all the meals, and while you didn't respond enthusiastically, you didn't give any sign that his hitting on you was unwelcome or annoying (and he was doing it hard enough for other people to notice).
In the future there are things you could try. Say no to offers for a walk or any direct overtures like asking you to dance (he'll get over it). Cut off conversations quickly, or pull in other people as a buffer. In a cruise situation, where you can't avoid him at meals, make arrangements with a few friends to help shield you at the dinner table (ie, making sure you don't have to sit next to him). Say no if he offers to dance, or asks to exchange information.
You don't need this for all (or even most) guys. Just the ones who pull the hanging around being attentive but not actually asking you out approach. If they ask you out you can say no, but it's harder to say no when he's just clinging in the hopes that you will respond positively. So that's why you need the friends as buffers/keeping conversations short approach.
As an aside - there are guys who can pick up on when a woman isn't interested and is just being polite. These also tend to be guys who realize that declaring love in an airport to someone you've known for a short time and never actually been on a date with is a bad idea. There are also women who are pretty clueless and honestly don't notice when a guy is expressing romantic (rather than platonic) interest.