I will start this out by clarifying that my group kind of cheesed their way through the majority of hazards by travelling ethereal, so a lot of the nice terrain stuff didn't come into play for us, which leads to me having no strong opinion to give on those.
I really appreciate that other than 7-11 this one actually provides the party with purchasable gear to make underwater adventuring possible if they don't have the required abilities themselves. The locations and encounters were fine, though I would recommend to switch out the nalfeshnees with wastriliths as originally intended; they are much more flavourful and fitting for the terrain they appear in.
The unique monsters, including both the underwater dragon and dagon himself, were very well implemented with a kit that is both flavourful and powerful.
Possible mutations are something I always enjoy seeing and the fact that they can be cured if necessary avoids feel-bads from players. My only major gripe with this adventure is that it doesn't provide any maps for grid lovers, leading to more work on the DMs side.
I would have liked more build-up for the final boss fight as well but overall it was an enjoyable module.
[4 of 5 Stars!]