The trip is on rain or shine! You will get wet, intentionally or otherwise, even if the sun is shining, so come prepared with the following:

Old sneakers or tennis shoes that can get wet for wearing in the boat. No sandals, as they offer no foot protection. Specialized river sandals OK.
Wool or polypro socks, if you wear socks.
Spare car keys (that you can carry and get wet)
Sunglasses with a strap.
Bathing suit (to be worn under clothes).
Waterproof sunscreen.
Insect repellant.
Broad-brimmed hat or baseball cap (that will not blow off).
Windproof jacket.
Rain gear, can be your windproof jacket (no ponchos, they are risky if you fall in).
Old wool sweater, or pile or heavier polypro top -- yes, even in summer!
Long sleeved cotton or poly shirt if susceptible to sunburn.
Water (at least 2 quarts) plus a short piece of cord to tie it into the boat.
Lunch that will keep in warm weather.
Waterproof storage for YOUR lunch, jacket, sweater, etc. Use a daypack, duffel bag or pillowcase LINED with a plastic trash bag. A trash bag alone will rip.
No coolers! Small insulated bags are OK.