Perhaps you should read the manual for that answer, Eric?
From the Icom
AT141 manual: "The antenna tuner can be conveniently installed both on the deck or in the cabin".
Also: "The AT-141 is housed in a durable, completely weather resistant case. You don’t have to worry a lot about rain, splash, and so on."
They also show 19 illustrations in the manual of mounting the coupler on a ship and on land. In all 19 illustrations, the coupler is shown mounted free outside of any structure.
While older versions of the manual mention a water drain screw and instructs you to remove it after installation
regardless of where it is installed (and instructs that installing it outdoors is perfectly OK), the most current
manual makes no mention of this drain screw at all. Perhaps it no longer exists in the newest models?
14 yrs on the top of our bimini with no water intrusion. We have the AT130 and SGC230, though.