http://www.acma.gov.au/ACMAINTER:STANDARD::pc=PC_2796
The above link has some info on C Tick. The way I read it is that items without the tick must not be hooked into a telecommunication system. I think a boat might be outside that parameter.
Any of the resident electronics gurus may be able to clarify what seem to be muddy waters. But I would be very gunshy of the opinion of a salesman at a boatshow.
If it was in fact illegal to import such items our good friends at customs would have a shedful & the word would spread quickly.