From experience, I'd take a guess at diode failure because it's been working hard. Alternators in boats, especially with LiFePO4 batteries work really hard and hot and the diodes are the first thing to go. It's normally only one though, which makes the alt noisy and reduces power, rather than stopping it completely.
It's worth carrying a spare alternator, they are not very expensive. When you have a failure change to the spare and get the faulty fixed or fix it yourself at leisure.
You can run the engine with the faulty alternator on place and belt fitted, but not if you've remove it and the belt.
MP