W.P.
Dear P.:
A life vest.
No really, anything that does not look sleazy and is comfortable yet nice will be good. Prepare for more draft and wet/cold air on the water. A boat is more informal than any other location (except for the nude beach). One word of caution though: Avoid high heels AND anything with metal or shoe nails on the sole. Get something with only leather or even better rubber under it. Not just to prevent slipping, but you don't want to scratch up the boat... Now THAT would make a bad impression. I would not worry about motion sickness, unless the boat can only fit four people. Bigger boats don't rock much, but check the weather channel for chop just to be sure. If it chops and you are out cruising, keep a hand on the railing near the sides at all times. Falling overboard is as embarrassing to the unlucky one as it is entertaining to the others. Have fun. ;-)
Regards,
W.