With a garden that size I would seriously consider looking into a drip irrigation system. That way you get the water exactly where you need it. These can be installed relatively inexpensively and easily. The rate of water can be controlled with a flow meter: essentially you tell it how many gallons per hour then set the amount of time. There are also timers with different zones so you can water specific areas more frequently. There are several resources for drip systems and flow meters. The flow meters can be installed with a regular hose but with a garden this large I would recommend the drip irrigation systems.

As for companion plants, if the plants are already of mature size then I would just leave them. The recommended companions for melons are corn, nasturtiums, radishes and peanuts. For potatoes, marigolds deter beetles. I would just at this point wait and see since the mature plants should not be moved.

Hope this helps. Karen