This game both sounds like a game I'd totally play and one that would be both fun and stressful to program, depending on how many things you include. I'm also a mostly-Inform person considering making a Twine game for this - I might be biased, but I think Inform could be perfectly workable if you keep it simple and, as JoshGrams said above, use some useful extensions like Hybrid Choices (which is great for keeping dialogue streamlined).
The premise also sounds neat - I love the idea of using the fox/hen/grain puzzle as a way to create different outcomes with cargo. Are you planning on using just items or also passengers? I can imagine different ways that could play out in storytelling with items-only, items and some passengers, and passengers-only.
I like the different captains idea a lot but I can understand why it might seem kind of ambitious for this timeframe. (Though writing for that also sounds like a lot of fun, TBH.) You could always figure out a restriction per captain maybe that simplifies or directs what makes each captain's interactions different - for example, if you had the 3 captains be at different points in time or times of year instead of having their actions affect each other's shared environment (although that might be part of the fun you're looking for). Alternately, it could be something like "captain 1: a local, everyone knows this person"; "captain 2: a stranger, no one knows this person"; "captain 3: has a relationship with either captain 1 or captain 3."
Lots of options and I'm sure it's mostly a matter of what you want to take on in the timeframe. Either way, it sounds promising.
ETA: would be happy to pitch in, though my resume is just "Inform and Twine tinkering and buggy games" and "boat- and ocean-related fiction interests."