There is NO excuse for not saying thank you for a tip. I ALWAYS says thank you when I recieve a tip! AND I mean it! I never count on tips, I base my spending habits on my wage so tips are gravey to me. I realize the ships staff count on their tips as income, BUT that does not excuse saying thank you! That is just a sign of a "burnt out" worker, or one with "bad attitude".
I teach my children to say "thank you" and "please" it is just good manners.