Get a passport.
Your photo ID and birth cert. are now inspected much more closely.
Name differences are questioned, e.g. Maiden name vs. married name.
Raised seals that are no longer raised, etc cause problems.
You lose it, and replacement is difficult.
A passport:
Is easier to carry.
Is immedate proof positive that you are a citizen of your country. This is important if for some reason, you need help at an embassy or consulate.
Even if cruise passengers are not required to have one. If something were to happen, (missed boat, etc.), that required you to stay overnight, or fly out, you could find yourself needing a passport to either stay, or get back off the island.
Chuck