Is it legally required to travel internationally with a passport?

I’m getting pushback on my 21 day notice. I plan to travel with license and birth certificate for a closed loop cruise, yet they keep requesting my passport.

Simple question: am I legally required to use my passport when traveling internationally or can I use my enhanced DL and/or birth certificate? Per the cruise, I can use the license and certificate, but I’m wondering per IML if I’m obligated to use a passport.

Thank you.