Shade, sunscreen, towels to keep in the cooler.
My kids can't tolerate "regular" sunscreen. They can only tolerate BabyFaces in Coppertone. Must apply it 30 mins before getting in the heat. Once they are hot, cannot reapply without chemical burning them (their pores are open when hot). Must allow cooloff time in the shade and cool towels to close the pores before reapplying. Colored zinc is all that works on their faces (the babyface stuff on the rest of their bodies).
If you have a cuddy, make sure it is well-ventilated and stocked with stuff to keep kid interested while cooling off for reapplication. If no cuddy, you will have to invest in a bimini top. Whether you like shade or not, it is criminal to keep a kid in the rays too long. I get precancerous stuff cut off me every spring from my childhood overexposure (I'm 38). They didn't know better in those days.
Be conscious of windburn on infant skin, too. Just choose a relatively wind-friendly spot for junior if you have to run hard for long periods. Obviously, aft is less bouncy, but I spent several years of happiness lying on boat cushions in the front passenger seat footwell of a 67 Correct Craft Barracuda straight inboard that was a vicious porpoiser. Loved it. Kids are durable.