Ice is a big problem on a boat. Never leave the dock without as much as you can hold. Even if it's a one day trip, it's easy to run through 500 pounds in a 15 hour day. I wish I had a dollar for every trip I had to come home early in my younger days because of a lack of ice.
Remember, the shaking and rolling of the boat is like shaking a margarita. The next time you hand shake a margarita, notice the size of the ice going into the shaker. After you shake it for 30 seconds, look again. It's half the size.
I call the loosing of ice on a boat the "Margarita Factor". It happens. It's a fact of life and you can never, never have enough ice on a boat. I like to have at least 1200 pounds on board before hitting the water.