One of the advantages of hydrogen is that tanks and fuel cells can withstand a large number of "charging cycles" much better than batteries. Additionally, for ships, the amount of energy needed to move is so enormous that I fear we'll have a hard time creating batteries that are feasible for long-distance shipping.
For short distance ferrying (including large, car carrying ferries) on the other hand, Norway has already implemented quite a few electric stretches. The major issue there is building the infrastructure to charge the ferries.
No they can't, the membranes of fuel cells degrade extremely quickly, as I a couple of 100 cycles before significant efficiency loss. That's currently one of the biggest issues with fuel cells and one of the biggest areas of research. Currently, batteries are far more reliable as an energy source.
It's true but the hope is to replace this with green hydrogen production through electrolysis of water. The idea behind this being, in a grid built on a large amount of renewable power there will be times (sunny windy days) with a huge amount of power overproduction. So you could run the electrolysis on all that surplus power and get hydrogen for it, instead of wasting it.
It's hard to say at this point if that idea is going to be successful.