We had our Vesper
GPS antenna installed in the
electrical panel right behind the unit for six years (until we
sold the boat) and never once got a "can't get position"
alarm other than the normal one immediately at
power on, which then went away within a minute or so after position was acquired.
I think, however, that this depends entirely on exactly where "below deck" is on your particular
boat, what the boat is made of, what is between the antenna and the sky, and a whole host of other factors that are very specific-installation dependent.