I realise that this is possibly not the right place for the thoughts I am about to express, being an aviation forum, but I am looking at it from the perspective of transport, rather than aviation. I am wearing my bullet proof vest!
I spend a lot of time on the two main islands and travelling between them, in fact spent two months commuting almost daily, so I do have some insight into this.
First of all the two population and commerce centres are St. Helier and St Peter Port, and most traffic (other than connecting which I'll come to) is to/from those points. The airports, whilst not far in actual terms, are, relative to the size of the islands, not close to either centre.
The current maritime service offered by Condor Ferries is inadequate in terms of frequency, and expensive. They have over the years put competitors out of business (one only has to think of HD Ferries most recently).
Isn't it time that the States Goverments put their heads together and provided a frequent and faster service operating at regular intervals throughout the day? I appreciate that there are some constraints imposed by tide and wind conditions.
As far as air traffic is concerned, there will always be a need for feeder services but there are ways of maintaining those and improving scheduling.
I don't have the MIDT figures to hand for the city pair JER-GCI but I'd guess from looking at the schedules that on an average weekday the seat offer for the 3 carriers is about 500 seats across 25 flights each way and in my experience occupancy is generally around 80%.
Is there a suitable high speed ferry to offer an adequate and cost effective service? I don't know much about maritime transport except that many people find it less stressful and more pleasant than flying, and arguably more environmentally friendly although possibly not on this route as the small twin props CO2 emissions are probably very low.