It's not 100 days, NASA already does 100 day missions on the ISS, there's little NASA doesn't know from direct experience that any navy could tell them. The most common scenario for a mission to Mars would involve 6 month trips with 18 month stays on the Martian surface, for a total of well over 2 years away from Earth. With a crew of maybe 4, on the low end. That's a situation that's completely unprecedented in history. Not only do you have extended isolation, but there's no way to get out of that isolation, no matter what happens. They are locked in to that extremely long term of isolation, there's no abort scenario to bring them back home sooner (in many mission designs anyway). The closest analogy on Earth isn't submarine voyages, it's actually prison. And that's a big reason why there's been a desire to study these sorts of things separately, without the extensive cost of space hardware in the picture. Because it is an unknown and it does have the potential to impact the potential success of a mission.
The Pitcairn Islands were settled by mutineers from the Bounty, and this settlement wasn't recontacted for 18 years.
Small groups of people being out of practical contact with anyone else would have been relatively common in the Age of Sail--it's only with the development of radio telegraphy that ship voyages were not horribly isolated.
How far have you looked? A quick google seems to confirm that NASA are talking to the Navy about this all the time.
Jesus. It only took them 110 years of submarine experience to realize a need for that? Well, that explains why the data wasn't available to NASA in the 60s…
Also submarines are a good source of data but not necessarily a good analogy.
Space exploration isn't the military, rank, duties, discipline and many other things like even the what drives the individuals are quite different.
A submarine would have a considerably larger crew than a manned Mars mission.
While a submarine has a very specific mission they are supported by the navy and their nation state, if they are caught in a bad situation they know that help is not only possible but would muster virtually every resource available to save them.
Submariners are selected first for their mental profile rather than a skill set and trained, astronauts are usually selected for their skill set first and then trained to adapt. In general there are more people that would deal well with isolation than there are world class biochemists and geologists that could adapt to such mental conditions.
Submariners have fairly basic duties, they are trained to be able to execute them under duress and the mental stress from both isolation and combat. Mars explorers would have to use their intelligence and think every day, you don't send them there with an SOP booklet to push buttons you send them there to discover so it's really important to understand how isolation affects their thinking process and creativity and what stimuli need to be created or simulated to keep their brains in working order to keep their creativity and intelligence up to maximize their effectiveness.
And you could probably find 1000 other different variations that make it pretty important to do these studies on the people like the ones that end up going to Mars rather than just say well the navy has done it and their peeps didn't go crazy so we'll be fine.
(This is far easier than trying to get it declassified)