it always gets colder than you think out there, so as mentioned above, some hoodies, maybe a Woobie or fleece blanket.

PLB PLB PLB, Oh yeah PLB

--Bright ass waterproof flashlight or strobe.
--sunscreen
--whistles
--umbrella
--flotation device for everyone (mandatory anyway)
--I personally enjoy swim goggles.
--inflatable pool flotations, especially if kids are in the boat. Lets face it the chance of a ditch is low percentage, however, treading water for more than 15 minutes in a cheap pfd sucks, especially if you have kids under 10 YO. A simple infant sized inflatable doughnut is super small and can be blown up in about 30 puffs and takes away about 80% of your floating weight & energy.
--some cordage. First you never know what needs to get lashed down. Second, it keeps people together.