What piece of gear would you tell a first time deployer to make sure to have?

Be sure to get lost of pictures with your brothers and be in them not just taking them.

Lighter! Don't care if you don't smoke!

A couple decks of cards

in our unit someone always had a backgammon board.

ammo, lots n lots of ammo. as much as you can stuff anywhere, even in your ass.

A GOOD quality knife. I carried a Randal Combat Survival knife for over 6 yrs in the service on deployment, in the field, just about everywhere except Japan and Okinawa. Carried it on numerous camping, hunting and fishing trips after I got out. Still have it, and the teeth on he spine are still sharp enough to slice deep at a light touch. No tool, gizmo, gadget, or piece of gear is as useful.

Music, a great book, a reliable knife a multi-tool and a small set of tools.

For us Squids we made sure to bring a comfortable pad to put on top of the 3 inch pad that was supposedly our mattress and anything else we can make like comfortable like flush baby wipes hate using sand paper to wipe. Laptop plenty of training films and movies, books we usually checked out from the ships library or someone would bring bunch on board during the on load and go from there. Anything from home that makes like easier when underway and in between port calls.

Not only a good knife, but also a multitool (Leatherman, Gerber, etc)

without saying the already spoken woobie.. If your going to be on a FOB COP or what ever that may have a shower then definitely a good pair of shower shoes.