Funny you mentioned these two franchises, because here in France, we had "The Return of Swamp Thing" renamed "Le retour de la cr?ature du lagon" ("The Return of the Lagoon Creature"). Talk about a mix-up!
The original Creature of the Black Lagoon is a movie I very much enjoyed, discovering it on DVD, then seeing it at the theater - and in 3D! That was amazing! So much better than the whole Real 3D trend (is this even still a thing abroad? Before the pandemic, I mean), that was really immersive. I watched the sequel on DVD, and liked it too. I saw The Shape of Water in theater back then and really liked it, but still haven't watched it again on Blu-Ray.
As for the Swamp Thing, I have blurry memories of the two movies. I found The Return at a discount store and remember I liked it at the time, but that was 15 years ago. The first one I found a few years ago, but can't remember much about it. I think it's time for a re-watch.
About a Golden Age swamp monster, I haven't read much of the Heap, but isn't he technically the first one, being born in a swamp?