Capetonian,
As a yachtie I know the Channel Isles quite well, and as a result my flying ( BAe photographer ) experience says 'fly every time poss! '.
The tides, rocks and fog make it just a matter of time for a ferry to come to grief, I for one wouldn't fancy being on a high speed surface vessel in a big wind over tide, and a few ferry happenings seem to bear this out.
I once made the mistake of taking a Condor hydrofoil from Jersey to St Malo, as it was too foggy for my boat ( pre GPS & radar, but I wouldn't set off even with them, relying on single systems - and dubious chart surveys - in such an area is a route to disaster ).
The hydrofoil went full pelt ( it has to, for it to work - a major snag ) in thick fog, relying on nothing more than a standard small craft radar to weave in among fishing boats and the occasional foolhardy yacht as we flashed past, playing 'Space Invaders' with lives.
The Channel Isles have plenty of money, but transport of SLF & small urgent freight is life-blood; if required, funding ought to be forthcoming for a suitable aircraft, maybe Trislanders with uprated engines and avionics by the sound of it.
Any airline operator who finds himself confronted by the cost of suitable aircraft would seem not to be doing his homework before taking the job on...