Where are you leaving from? If it's
San Francisco, the first few days (and nights) may be quite cold and wet. You will need several layers of thermal gear under your waterproof outer layer. If you're leaving from Los Angeles you can probably get away with one less layer, but it's still going to be colder at night than you expect.
You want waterproof tops and bottoms, and it's good if the pants are more like overalls so the
water stays out. West
Marine has some decent low-cost gear. You don't need a lot of warmth in the foulies (as long as you bring the thermal gear), and light-weight ones will be welcome when it gets hot for the second half of the trip.
Bring sunscreen, earplugs, sunglasses, and seasickness medication that you have tried before casting off.
I really like a lifejacket with a built-in harness (Mustang, Spinlock, and WestMarine make good ones), and I *strongly* recommend that you wear a proper tether and stay clipped on. If the boat has
jacklines (you clip your tether to these) I suppose you can do without the
lifelines. If the boat doesn't have
jacklines that's a bad thing.
But I have to ask, is this boat (and skipper) properly prepared for this
passage? You should look at the
safety gear required by the
Hawaii races (see the
Pacific Cup Knowledgebase for examples). You probably don't need all of this, but you should discuss it in detail with the
skipper. If you aren't comfortable with the answers, this may be a bad idea. At least have a knowledgeable ocean-sailor take a look at the boat with you.
This can be a wonderful experience with the appropriate boat and crew. What type of boat and how many crew?