In a 2 prong polarized outlet, and even in 3 prong ones, there's a brass screw and a silver colored screw for electrical connections. The electrician will wire the black wire to the brass screw and the white wire to the silver colored one. That's why they're different in the first place, they're only supposed to be wired one way (thus the term polarized).
Most power transformers you see on the pole have only one insulator, that's because only the hot line is the power feed to your fuse/breaker panel. Only the black wires come from the breakers. The whites only go to a common neutral buss bar in the panel.
It's true that a break anywhere will shut off the power, but the black is the power feed line.
If you shut off the (hot) black line, there's no power anywhere to the cord.
If you shut off the (neutral) white line, then the cord can still be hot, and a simple ground to a case edge or water puddle anywhere along the cord (after the relay) would start a live circuit conducting to that ground....zappa you booty.
That's why it's best to switch the black wire. It won't effect the performance of the circuit, but it does effect the safety of it.
If your pump plug has both blades the same size (never seen one but it could happen) then put the relay on the non-ridged one, and always plug that non-ridged side into the smaller right hand blade side of the outlet to keep your pump polarized.
Be safe, live long, watercool like a madman.
