In folktales, trolls traditionally live under bridges. Why is that? It makes the most sense if trolls are amphibious creatures who paddle around in the water between their feasts on merchants, PCs, and billy goats.
Making all trolls amphibious makes sense of their hairless, rubbery, froggy skin and their weakness to fire. It also lets them catch some of that “Zombie Survival Guide” horror: you can’t escape from them by land or sea. In fact, trolls are much scarier on the high seas: they’re nearly invulnerable to fire under the sea and on dangerously flammable ship decks.
Get rid of the scrag, the D&D aquatic troll variant. Instead, give all trolls a swim speed and water breathing. Put troll dens near rivers, lakes, and bogs, where they can retreat, Grendel-like, when they need to regenerate for a bit.