I am also a happy camper when it comes to 6X9's in boats. I do believe that 6.5's can sound a little better. However, 6X9's are louder when powered sufficently IMO in a boating application.
My Infinity Kappa 693.5i's are louder than my Infinity Kappa Perfect 6.5's despite getting less ampifier power. (130RMS versus about 90 RMS respectively)