Keeping crawfish alive overnight?
Oh you're fine. For maximum freshness, follow the advice from (this one is better, no spurious exclamation points!)  this online FAQ

How do I keep crawfish alive until I boil them?
Keep them in the sack and cool with ice, but not submerged in water. A large ice chest works great with the drain spout open and the lid cracked for breathing. Properly wash the crawfish by submerging 2-3 times in clean water. Let them soak for 20-30 minutes the last time. Use of salt on final cleaning is best.