Sump Pumps are expensive, but marine bilge pumps without a safety off are cheap. They're like $15 on amazon, and the failure mode is burning out their own engine. Put a 3/4" hose on the output and wire them up to a car battery or 9-volt.

I designed a backup grey water fountain system for a burning man camp based on them, but the primary didn't fail so they weren't tested out.