As an example, think of a sailboat as a closed system (that's actually pretty easy to do). Everything that comes aboard will eventually need to be removed. Unless, that is, it is destined to become part of the boat, and this has to be a very limited set of things, or we will sink!
I think I first had this realization early in my liveaboard career, when I was cleaning the floor. Anything I missed was eventually going to find its way into the bilge, and then overboard via the bilge pump. But bilge pumps are kind of finicky - for example, they don't like hair at all! It gets wrapped around the driveshaft and the impeller, eventually bringing the pump to a halt. I started viewing the cabin sole as the last-ditch bulwark keeping hair out of the bilge. The hair still must leave the boat, but hopefully now via a full shop vac.
The closed nature of a boat system is nowhere more obvious than with garbage. Every bit of packaging (and make no mistake - packaging is by far the bulk of it) must first be carried down the dock to the boat, and then carried back up the dock to either the garbage dumpster or the recycling dumpster. Why do we have so much packaging? Cellophane wrappers over cardboard boxes over plastic internal bags.
(Aside: Thank heavens our marina provides a recycling dumpster. It seems that it fills up nearly twice as fast as the garbage dumpster.)
It seems that every time we head up the dock, there is something to be carried up to the dumpsters. There is no dockside garbage service - there's just us. More than once I have wished that there was a sheltered station by the recycling dumpster where I could remove the unnecessary layers of packaging before hauling things down the dock. There would be less to haul (two ways!) and there would be more room aboard. Or better yet, how about locating that station at the grocery store?
This is another instance where living on a boat brings the consequences of your lifestyle decisions directly to you. Realizing that I've come at this obliquely in some other postings, I have created a new tag, footprint, to flag them.