Do a quick search for coffee based developers.
you can print on leaves. Or make a paper sensitizer based upon flower/leaf pigments.
Seawater makes a fine hypo clearing agent (used by US Navy in WW2 and since...why waste fresh water until final rinse? the basis for the product "permawash")
You may be on an island with a volcanic vent (sulfer based compounds)
I guess you are presuming that a crate full of film washes up regularly?