Good points there, Doctor Q.
I think you have to look at it like this someone's willing to pay you to paint a picture, and they want something that looks like a well known cartoon character. The reason they want a picture of that character is because the person or company that created it has spent a lot of time creating it, and a lot of money on creating and marketing it, and that's what's made it well known and that's why people are willing to pay you for doing your paintings.
If you were just painting a simple sailing boat like the link you provided, it's unlikely anyone would buy it. The reason that it's desirable is because a company has made the show popular, and because of the work they did in making the characters (and style) popular. That's the reason for licensing fees it's a way of paying back the original creator for the work they've done to make the work you've done more desirable.
Now whether you choose to worry about it is up to you. I've made t-shirts with hand drawn characters on them for individual gifts, and I personally don't have a problem with that because they're just silly little things I do as one-offs maybe once or twice a year, and I wouldn't do them for money or very often. As far as my morals go, I would always try and buy the original t-shirt if possible if it was from a less well established creator rather than making my own one.
As other people have said, it's probably unlikely that you'll get caught, but the larger scale you do it on, and the more money you make, the more likely that is, and perhaps it's more how far you would want to go morally.