First choice:  “Margaritaville”
Why?
You can count on it being the exact arrangement as the original recording and in the same key.
Usually, everybody chimes in on, “Salt, salt, where’s the damn salt?!”
Some folks are actually drinking margaritas.

Other songs where the crowd is on your side:
“Old Time Rock and Roll,” “Hey, Bartender,” “Lean on Me” and maybe “Bye, Bye, Miss American Pie” with all of those lyrics.

You do a halfway decent job,
Staying in tune and not screwing up the lyrics too much in your somewhat inebriated state,
And people you’ve never met before congratulate you on your performance.

OK, maybe a stretch here, but cruise ship karaoke might just save the world.