I wear my tri-shorts and an Under Armour heat gear baselayer under the wetsuit (though if i'm using my sleeveless wetsuit, i'll just wear the shorts). If you're already wearing the baselayer top, you may not even need the jersey (i usually don't bother putting one on nowdays). Gloves and socks are optional. I always put socks on (because i'm wussy like that ) but nowdays i'm thinking i'm just going to use gloves if it's cold (i.e. under 70 degrees because i'm skinny). Sunglasses are highly recommended.

For a 12 mile ride, 1 water bottle should suffice. I also recommend carrying the the minimal stuff to fix a flat (at the very least a spare tube, patch kit, levers, and a C02 inflator or minipump that easily goes to 100 psi, or both). Having to walk 7 miles back to the transition area or trying to flag someone down to give you a hand isn't very fun. Another good thing to consider carrying for race days is a Vittoria Pit Stop. It's fix-a-flat for your bike: http://www.trisports.com/vipitst.html I carry one now because i've never flatted during a race and i feel my good luck is starting to wear thin.