I'll be bringing my passport on an upcoming cruise as I know it's the preferred form of ID. But I would rather not bring it down with me every time I'm on a shore excursion. Is a copy of my passport just fine to reboard the ship on NCL, and for the Bahamian government if anything (knock on wood) was to happen?
If I remember correctly, on Royal Caribbean you just need your sea pass card to get on/off ship. Nothing else.
Empress of the Seas...May 2002
Enchantment of the Seas...April 2004
Sovereign of the Seas...April 2005
Explorer of the Seas...June 2006
Majesty of the Seas...July 2008
Majesty of the Seas...July 2009
Explorer of the Seas...August 2009
Majesty of the Seas...May 2010
Explorer of the Seas...July 2011
Enchantment of the Seas...August 2012
Grandeur of the Seas...August 2013
As said, getting on and off the ship is no problem. The potential issue is getting stuck in a port and having to fly back to the US. You would need your original passport if that happened. Not likely, so it depends on how you prefer to handle that kind of risk.
The last couple cruises, we have actually been asked for photo ID as well as the sea-pass card to re-board the ship in port. We copy our Passports and any other pertinent documents like health insurance, and put them in a plastic sleeve to take in port and leave the originals in the safe. They will make you a copy at the ship's desk if you forget.
The copies would be fine to bring ashore, sometimes they will ask that you carry a picture ID, in that case I carry my drivers license and keep the passport in the safe. I've never had to use the ID, but have had it on me.