There are 39 hydrogen refueling stations in the US, all but 4 in California.
There are tens of thousands of places that sell diesel in the US. I can stop at a gas station with a diesel can, fill it up, and take it to the boat. You can't do that easily with hydrogen.
Hydrogen fuel costs significantly more than diesel. 9600 BTU/dollar vs 55600 BTU/dollar.
A hydrogen fuel cell / tank / motor is more expensive than a diesel engine setup.
I can work on my diesel; if it is beyond me I can easily find someone to work on it. How easy is it to find a hydrogen fuel cell technician?
You need a big freaking tank for the hydrogen - the tank is 5 to 10 times the size of the diesel tank (for equivalent BTUs, a hydrogen tank at 5000 psi has to be 9.5 times the size of a diesel tank, but a hydrogen fuel cell is more efficient). Since there are few places that sell hydrogen, you need a big tank to make it between fill ups.....
Speaking of axioms, there is one that says something to the effect of "Early adapters are often left hanging".