Seriously, drowning them is probably more humane than poisoning. If you want maybe electrocute them first before you drown them, or start a carbon monoxide chamber outdoor, but that create some safety risk for yourself too.
At work we trap critters and toss the havahart trap in a lake with a rope tied to it...when the bubbles stop, you can pull it in. We tried neurotoxin in diet Coke but they didn't always get a quickly-lethal dose and would stumble around like they were ill or rabid and it freaked people out.