Wow... sorry, but I really disagree. Keeping water in the bottom of your container sounds like a really bad idea to me. You do not want your soil to stay wet all the time. I think bobber is talking about trying to make a homemade self-watering container. But in self watering containers, the soil is NOT in contact with the water beneath, they are separated, with just a wick connecting the soil to the water underlying. Or in some cases nothing connecting them, but some pores in the barrier between so water vapor can rise in to the soil.
Even so I don't really like growing in self watering containers, because it seems difficult to manage the soil moisture that way. A lot of things like to dry out a bit between waterings, but the swc doesn't allow that to happen. Just water your containers when they need it; not that hard.
I haven't tried the gravel mulch, but 1) it doesn't break down to feed the soil and 2) it seems like even white gravel sitting in the sun would tend to heat up.