That's exactly right. When it's out of the water, resistance is VERY HIGH. Basically the two contacts aren't connected. However when the contacts are in the water, the resistance is low, meaning the two wires are connected. There is probably a lot safer way to do this, but it really depends on the relays used. I'd love to get someone else's opinion on if this'll work or not. I'm just speculating here. Anyway, here's the very simply schematic thing I drew up.
As you can see, you have power from an outlet going to the switched part of the relay. You also have less power (depending on the relay used) going to the switching part of the relay. (Coiled part in the picture). When the switch is connected (contacts are underwater), the coiled part of the relay makes a magnetic field which causes the switched part of the relay to connect, therefore running your pump. When the contacts are out of the water, there is no magnetic field, and therefore the circuit that runs the pump doesn't have any power. Everything is connected to a common ground. I'm sure you can find an aquarium safe water switch and choose your relay based on what kind of power it needs. If you want to find out how relays work, go
HowStuffWorks "How Relays Work".
I'd love to get some feedback from some electrically talented aquariust out there.