I just back from a three day aboard the Peace and loved it but back to your question:
Non clothing items sunscreen, brush, toothbrush, mascara, chap stick, sunglasses, hat, money to tip the crew not to play poker, and two towels.
Clothing items: I brought two pair of sweats, two jackets, two pair of socks mittens two bathing suits. Ofcouse changes in underwear. A couple of t-shirts. The key is layers so if you get hot you can just pull one off or if its cold add another.
During the day you live in your wetsuit or drysuit. And for me my wetsuit dosent come off till after i am done diving for the day. so i like to be warm and comfortable. so layers layers layers. It gets cold on the boat at night and in the morning.
You are sure to love it... As a side note eveything but my towels fit in a medium sized duffle. I wore my jacket so that wasnt included in the bag.
Have a great time.