Re: We did it! We took our 3 week old sailing today!
We sail with our five-month-old daughter, Zoey. She's usually in someone's lap, in a car seat identical to MedSailor's -- but our favorite place to put her is her booster seat.
Her seat is the "Fisher-Price Healthy Care Deluxe Booster Seat." (can't post links) It has a three-point harness to hold her in place, a strap that we put around the companionway ladder's rail, and a tray for holding her toys. We strap the toys to her harness with pacifier holders.
She sits there facing aft, watching us at the wheel. It's a ton of fun.