The temp-sensor is actually a thermo-switch, it's either on or off. When it activates at 235-degrees F, it's ON and grounds the wire on the harness. That's what activates the relay. So in the normal state, or disconnected altogether, your water-pump shouldn't run more than the 30-seconds that is determined by the timer in the relay.
So if you disconnect the plug from the sensor and the water-pump stops, then the sensor's bad. You can get a new one from NAPA auto-parts:
Turbo coolant switch.
This is a lower-profile unit that works with large turbo upgrades.
However, if you've already disconnected the plug from the sensor and water pump still runs, then you've got a short in that wire somewhere. Trace all the way back to the firewall and see if it's grounding on a metal edge somewhere...