Here's a couple of favorite cool drinks aboard Atom when we're in port and have some ice in our portable cooler. We use a standard household blender that runs off an inverter. 12 volt blenders are nice to have but they cost more and are not so easy to replace if they break during a cruise.
Chocolate Banana milkshake (makes one large mug full)
1 ripe banana
1/4 cup water
3 rounded tbsp nonfat powdered milk
1 rounded tbsp pure cocoa powder
dash of cinnamin or vanilla extract (optional)
2 cups ice cubes
Blend and serve. Use more or less of any of the above ingredients to suit your taste. No sugar is needed to sweeten the cocoa if the banana is ripe enough.
Watermelon Juice (makes about two large mugs)
Fill your blender about half full with seedless watermelon pieces. Add a cup or two of ice cubes. Blend well and serve.