Well, you've got some really excellent responses.
I'm going to assume you'll take a camera. Don't forget your charger and spare memory cards. If space will allow, take a very small laptop to make sure you get the pictures. I would suggest taking a least one card for each day of diving. If you have a camera, don't forget silica gel to put inside the housing.
But as for liveaboards, you'll be spoiled for life. You've heard the eat/sleep/dive adage. It's true. It's awesome. You are on the sea. You can smell it for a week, and it's awesome. The rocking of the boat is like a cradle, lulling you to sleep.
Make friends with the guests and crew.
Above all, listen to the dive briefings very, very carefully. The guides know what is where, all of the pitfalls, etc. Pay attention to all of the safety stuff.
Above all, have a blast!!