Mechanics question: how do you decide when something is definitely dead?
In a 'when is it mechanically dead' or a 'when it is dead' kind of way? For the first, if your health falls below the red line (below badly wounded), an instant endurance/toughness/not dying test is rolled. If you pass (right now a coin flip could be used instead of a die roll, fixing that to allow for variable results though) then you're alive but in a pretty bad spot. If you fail, you're dead.
At least, that's how it is for NPCs.
For PCs, if you fail you're still alive but just barely. Its a sort of grace period, basically, and allows you to do things you may not normally be able to do. You can progress your turn as normal, but at the end of the turn another survival roll is called. If you fail that, you're toast. So you normally want to deal with the whole 'bleeding out/edge of death' rather rapidly. The actions are death defying actions, and they're listed on the first post under the mechanics spoiler.
Basically, you can try to flee but every enemy who can will try to attack you, you can try to attack every enemy within range but if you fail to hit (damage is optional) then you die, you can do an action that's automatically a critical but you die afterwards (so using it to heal yourself would be a bit of a waste, since you'd do a bang up job but still die). There's also the fourth, but that's only if you're brought to a dying stat due to your own fault (stepping out of an air lock, swallowing a live grenade, drinking unknown chemicals, etc).
If you meant how one can tell when something is really dead, then I'd say when there's pretty much nothing left of it. Most tend to die far before that point though.
Important question - are there countryside-style areas, or is Ship City a metropolis throughout?
The outskirts would be the best bet, although if you want a quality looking countryside-esc place that would be uptown, or the rich district. Of course there its more a really well maintained park/yard than a countryside and barred off to most. The outskirts used to house a rather nice and (relatively speaking) large park. Its still there, but its fallen under poor maintenance and has become a bit of a meeting place for unsavory types. There's been some talks about removing a good chunk of it for a good and proper embassy (don't want to get shown up by the other shipstates after all), but that's rather fallen to the wayside due to recent events.
So for the most part the whole of the city in the ship is a metropolis, but you'll find a few parks (the most notable in the outskirts and uptown) here and there. Anywhere beyond the two notable places would be very small or oddly designed (a building housing a park, with each floor a separate part of it, for instance) though, considering the need to be as efficient with space as they can (unless you have the money or clout to forgo that).
As a note, a few things: I've determined (a while ago so I'll have to rewrite it in full but I still have it in my head) a few changes for guns, time, cover, and a small change to food.
1) The basics is that guns can be more varied now (differences in base (determines the weapon size) and receiver (determines the size of the round)) and the designing of fun rounds, attachments, and hopefully some fanciful stuff like names. There's some other changes too, such as recoil and how many rounds one can fire in a turn and how that works.
2) The problem with time now is that there's a lot of it wasted. You've probably noticed that if you talk to someone while another person is out exploring, you've pretty much lost a turn you could of used otherwise. I've decided to fix this with what I'll basically call a token like system. Before, if that happened not much could be done. Now, you'll gain a mark/token/what have you to indicate that you've time to make up.
Those tokens/marks are used automatically during a few occasions (when you're not temporally linked to another person who doesn't also have a token, it makes logical(ish) sense, and you're doing an action that would move the game a turn ahead). If that happens, you're action is truncated to a free action that doesn't progress the clock.
It won't completely get rid of wasted turns, it should put a dent in it and make the time go by at a more meaningful pace. It also doesn't push people into different time lines (much), but rather just makes it more flexible. All of this is mostly a behind the scenes type of thing, but I figured if I told you there would be less confusion when someone's action becomes non-time progressing.
3) I knew guns would be overly deadly (entirely possible to down someone with a hand gun in a single turn) so I knew I had to have a good cover system to allow for gun fights to not just be (too much) a match of rocket tag. Basically there are varying sizes of cover (partial, half, most, full) that each have two stats (avoidance and threshold). Each cover is given a point in each in alternating fashion, starting with threshold. So Partial has a threshold of one (imagine if you have one armor, basically), Half a threshold of one and an avoidance of one, Most a threshold of two and an avoidance of one, and Full having a threshold of two and an avoidance of two.
The first stat is added to your roll to avoid a ranged attack so that's pretty simple, the second works similar to armor but with a few added rules. Each cover is given a threshold number depending on what you take cover behind (a paper wall would give you full cover (provided you don't peek your head out) but while it would have a threshold of two, its threshold itself wouldn't offer much of any protection).
4) You can now burn a point of food/satiation to recover from fatigue on a one-to-one basis. Gives satiation a point beyond keeping a high health/regenerating health.
Don't worry if the above doesn't make a lot of sense, I'll put it in clearer terms when I get everything written up again.