[aeroman]

Water provides the 'energy' that drives many storms. As a bubble (or parcel) or air rises, it cools. Eventually it gets cold enough to condense water (some of which forms rain). This heats the parcel of air, making it less dense than it was and so more buoyant - so it rises faster and invigorates the cloud/storm.

If you have more water around, this process is stronger (as you have more available water to condense and energise the storm), eventually producing more rain.

This is the reason why you see the biggest thunderstorms in the tropics, where it is warm and moist.