it's best to keep the cable run as short as possible.

Which is usless advice if it's a network-connected GPS antenna. NMEA2000,
SeaTalk, etc, are fine with longer cable runs.

You don't mention if this is a power or sail vessel.

Generally you want your GPS unit mounted where it's going to have an
unobstructed view of the sky. Putting it next to a radome might be a
problem in that the signal would be getting blocked. Not enough to entirely
lose all fix but enough to degrade the overall accuracy of it.

-Bill Kearney