I'm someone who loves to collect currency, rare coins, foreign coins and bills, etc. While I like the idea of handing out $2 bills or presidential dollar coins, etc., I realized that if you're outside of the US it's an oddity that causes inconvenience for the other party. Sure, they may smile as you hand them a $2, but in the back of their minds their thinking how much of a b*tch it'll be to deal with it.
If you're in port, their local bank may or may not take it, so they are stuck with trying to give it out as change to some other American tourist. If they are crew on the ship, they have to figure out the best way to deposit it so that they can send it back to their family in their home country.
So sure, if you want to tip that way, feel free. There is nothing stopping you. But don't believe that their smile doesn't mask their hidden annoyance in now having to handle the problem that you just gave them.