Tom Selleck -- Sorry for Stealing Water? We Drought It (TMZ TV)

Tom Selleck's forking over $21,000 to put that embarrassing stolen water caper behind him. Shockingly, a P.I. busted Magnum jacking H2O from the state ... just so she could keep his avocado trees in the green.

The cash is nice and all -- but a better settlement would've been free guac for life!! Just sayin' ...