depends on the source.
In rice fields and swamps, when I was a kid, we would take a piece of plywood out there about thigh deep and stand on it. After about 10 or 15 minutes you could feel critters crawling on your toes.
Then we would hop off the plywood and it would slowly rise to the surface, the crawdads would never know until the plywood breached the surface and we would scoop up about 20 or 30 big ones and toss in the bucket.
Shrimp & minnow traps work good in creeks and such. Frozen canned cat food works good for bait.