Re: Interest Check, Hash out details, Recommendations, and stuff
here's another idea. it could either be a continuation of the above after you finish with those (or if the players fail and they all die), or you could just start one with this and run.
instead of a living crew transported through time or some such, you could play the crew of a sub lost at sea. you could even make up your own modern day or future sub so you (and your players) don't need to know any details of how past subs were lost if you don't want to.
Concept:
you're doing your typical day-to-day routine, not aware you were lost at all. then the call comes in on xmas, which is a first, and the players start putting it together. once they've rp'd through the initial discovery of their prior demise, they realize the barrier between this world and the next has broken down as more calls start coming through. the new calls are not directed toward them, but rather open calls back to shore command for help. i'd skip the whole code thing and just hand wave it as "they're ghosts and the transition allows them to communicate with those they have a connection with".
something supernatural is happening in modern day. maybe ghost-like enemies, maybe demons, maybe something else. regardless, modern day living crews are being overwhelmed. enter the players. as they're on eternal patrol, they still feel a duty to answer the call and protect their brothers and sisters at sea.
pros of this approach is that you can totally run it as a regular modern day navy game. the fact the players' characters are dead doesn't necessarily need to come into play. they can "kill" the opposing supernatural forces, and the enemy can "kill" them. the ship and its armament are eternal. thus torpedos and such are recreated automatically, perhaps over time or maybe they're never "expended" when fired. that solves the problem of needing resupply. on the other hand, you can also take advantage of the characters' ghostly nature and run some pretty cool supernatural elements as well. maybe they need to board an enemy's vessel. they could silently bring their sub up underneath and crew members could exit through the escape hatches (unaffected by the water or depth) and pass through the hull of the other ship. you could focus on the ship combat, and/or focus on the players discovering the advantages of being ghosts and what that means for hand to hand combat, espionage, etc.
in short, all the crews on eternal patrol become the front line versus this new threat, still protecting their country and families as they always have and always will.