I Know!
#1) Surfboards to ride on the water
#2) Goggles as hats
#3) A real, big, beach with many towels, umbrellas, etc.
#4) Hamburgers and Hotdog stands
#5) A new, improved water gun that does 8 damage and looks like a different type of water gun (than the yellow one from last year)
#6) The place off-map that the boat leads to, the place can be an island with the beach on it and a cool city
#7) Some sort of Cruise Ship
#8) Speed Boats, Jet Skis, etc.
#9) Make water realistic where you can't stay underwater for a long time. This would make people have to buy scuba diving masks or air tanks that actually work