In my fantasy world, there has been a longstanding presence of underwater civilizations, both alongside and independent of surface-based kingdoms. In the style of Tolkien's Beleriand, various continents have risen and been destroyed over the ages, but the underwater nations (while occasionally devastated by said rising continents) have mostly persisted.
In light of this, I'd like to develop at least one underwater language, whether ancient or otherwise, that can influence my other conlangs as a longstanding, widely-spoken tongue. Sort of like Latin, but with mermaids.
This brings me to my question: What spoken phonemes are most easily recognizable underwater? I imagine some sounds would travel farther and be more distinct, and I'm sure that would have some impact on language, but I'm not sure exactly what sounds, or how much. While I'm not 100% concerned with realism, I'd like my language to be rooted in sounds that could feasibly be used to communicate underwater, with minimal handwaiving. My mermaid species is amphibious, so they need to be able to speak above water as well. Some of them are even part of surface-based civilizations, and speak the languages of said nations, so they can't have an entirely different set of available linguistic sounds (e.g. they can't just sound like dolphins).
EDIT: I meant to mention that answers can feel free to make any necessary assumptions about the vocal construction of said mermaids / merpeople, provided they can still conceivably speak ordinary, above-water languages.