At what wattage does a pond pump pose a danger to human life?
The rated wattage of the pump won't really make a difference. If there is some sort of fault, it will likely allow whatever the supply has to give to flow. The solution you are looking for would be to make sure the supply you are attaching the pump to is ground-fault protected, by either having:

A GFCI-protected receptacle (GFCI built into the receptacle, or into a down-stream one connected through the LOAD terminals) or
a GFCI-protected circuit breaker

This will make it so that if there is a failure in the pump, the GFCI device will detect the failure and shut the power off.
If you are really worried about it, you may also look into low-voltage pumps. I am not sure if these exist or not, but I think this would be inherently less dangerous. However, GFCI protection is generally accepted to be perfectly safe for these types of uses.