Chuck down at least sixteen feet of water? (4,4,3,4)
I believe the answer is:
rain cats and dogs
I believe this clue is a double definition.
'chuck down' is the first definition.
(both can informally mean to rain)
'at least sixteen feet of water?' is the second definition.
(at least 2 cats and 2 dogs is a total of 16 feet)
(Other definitions for rain cats and dogs that I've seen before include "come down hard" , "bucket" , "To fall heavily" , "Pelt" , "Pour" .)