Any DC-powered, low flow-rate, inexpensive water pumps?
A/C submersible aquarium pumps are designed to work underwater, why are you worried?  I've used similar pumps in fresh and salt water for years and never had one encounter an electrical failure.

I have tested these  submersible DC pumps with fresh water and they seem to work OK. Haven't tried them outdoors yet, but they are fairly quiet in case that matters. They only claim 6.5 feet of head, but that site has other pumps that may be more suitable.