Of those 3 ideas first one would be harderst to pull off. Since as Savin stated on discord demons if there have been some sort of Poseidon-like god(ess) it was their first aim for godslayer rampage. So now pretty much only cuz Marae is at lake it still exist. On the other hand it's odd since...she's godess of nature not specificaly waters so how come she stopping lake from been meet same fate as ocean(s) before
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Ah, but a Poseidon-like God/ess would care more for the seas
... that's why I suggested the Lake, which would be more between the interests of a wood/nature God, and not picked a shore or sea location (and since the place is already there, I guess it save coding time)
(to put an example: Dryads and Nymphs are nature creatures, Dryads are land spirits -
specifically from trees in general, be from woods or a forests- and Nymphs are in-land waters creatures, like lakes and rivers; both would be under "protection" of Marae -
which is basically a super Dryad-. Essy it's another example of a creature that would fall under "Dryad" cathegory.
The rough equivalent for a sea creature of a Dryad would be a Siren.
Also, it's very clear that the Lake is half corrupted/polluted, it could lead to some quests, or simply be kept as that and allow for strange encounters. -
this could get even more developed depending on Marae's factory quest, tho could be left as is-
Resuming (excuse my ramblings): a Goddes of Nature like Marae would have land and small waters under her sphere of protection, but doesn't involves the seas, which are a completely different environment in itself.