There are countless little communitues that rely on goods being flown in or coming in by tugboat
I applaud the OP having a dream on the water, although, it sounds like there is insufficient capital to give it a go and it does seem like a stretch to be viable. A shoestring, family run, single cargo carrier isn't going to be very reliable, let alone likely profitable. It's described as a very small carrier, which would be inefficient. Sailing isn't free. The sails for a vessel of this size will cost tens of thousands of dollars, plus running and standing rigging, and wear out far more often that a weekend warrior's. Down for repairs is one thing, but I'll bet it won't be that much cheaper to operate per mile than a slow displacement diesel barge, when you factor in the life cycle of the sailing bits.Things like clothing and electronics, that don't need to be flown in and could be made cheaper might be an option for you?
Reminds me of a moment I had once. Wife and I were going bareboat cruising for a week in the Caribbean. Then I coincidentally needed to be in Puerto Rico for the follow week to work. I didn't want to drag formal work and evening wear around on the bareboat, so we decided to ship our work luggage directly to the hotel in PR, so it would be waiting for us.The traffic carried by ships cannot be diverted to rail, or vice versa.
That's perhaps the right attitude to start with. However, you'll also need to skills (and tons of energy) to turn them around, as every disgruntled customer will talk ten more out of doing business with you, even if they're wrong.I learned along time ago you can't make everyone happy, just have to have tough skin