A lot of people use timers. Since you mentioned a double pole switch I assume a 240V circuit. Home Depot / Lowes / etc have timers. Intermatic is a popular brand. I looked recently for water heater timers and they were under $50 I think. Easy to wire in. I actually have two, one for the pump and another for an auxiliary circuit. I also wired in a couple single pole double throw switches, one to bypass the timer and force the pump on, and another to keep the the timer from turning the pump on...forcing it off. I KNOW...CarlD will kick in about local codes. Where I live nobody cares but it is something you have to consider if it is an issue.
Al