Does it still do it when EVERYTHING else on the boat is switched off?

You may have something that's electrically noisy (laptop, tablet, charger, inverter, etc) that might be hitting that frequency with some interference. Hence the scanner stops at that channel.

Also does the scanner have a way to mute that channel (ie exclude it from the scan)?

Unplug the antenna and see if it still does it. That will tell you if the bug is in the radio or an actual signal.

Another simple diagnostic test might be to buy yourself a hand held marine VHF which are very handy as a backup and see if it does it too. If yes then you probably do have a stray signal.

LIke nebbian said theres heaps of interference sources these days because of switching voltage regulators, they are even in some LED nav lights and probably in your solar regulator if its an MPPT type. In theory they should meet a standard where RF emissions are so low they dont effect radios but no one tests them I knew someone whos radio got noise across all channels and it was from a new masthead light with a switching voltage regulator in it.

If the interference is not coming in through the antenna, which can be proven by unplugging it, then there is a fault in the radio.

The receiver should only be detecting signals detected from the antenna plugged into the antenna socket. If a signal gets in any other way or is internally generated there is a fault in the radio or at best a poor design. Superheterodyne or not.