Well, you're right to be cautious.
For leaks, since they can come from any point, Conformal coating is still, IMO, your first defense.
Otherwise, a flow sensor, and a temp gauge (in the water) is my second step, while letting the mobo (with OCP) protect the CPU.
Hose clamps are a life saver
Otherwise, to detect water, you can use the old salt pill in a clothes pin: make contacts there, and put some salt: when the water dissolves, the contacts trigger.