In a nutshell, Aventi are essentially D&DAtlanteans: the descendants of an ancient and mighty human empire that, when sunk into the ocean, magically adapted and survived by turning into amphibious, but otherwise near-perfectly human (pale-blue skin and tiny fins on the forearms and calves aside), creatures; essentially being more humanoid and amphibious merfolk. Season with elf-style arrogance and a conviction of their own superiority —despite usually coming from a place nobody has heard about—, and you have them in a nutshell.
If you want to play an Aventi in 5th edition, it's really easier to just grab a Triton: the Triton's stats do pretty much everything you want out of the Aventi, and they even match up in terms of fluff and, mostly, experience.
Amphibious (Ex): Can breath air and water with equal ease.
Base land speed: 30 feet
Swim speed: 30 feet
+8 racial bonus on Swim checks to perform a special action or avoid a hazard, can always take 10 on a Swim check, can use the Run action whilst swimming in a straight line.
Water Spell Power (Ex): +1 caster level when casting spells with the Water descriptor.
Human Blood: For all effects related to race, treat an aventi as a human.
The basic idea of the Aventi was, of course, re-used in Pathfinder with the oh-so-imaginatively named Gillman. These guys are almost identical to Aventi in every way that matters, bar being somewhat more cosmopolitan, interacting more with surfacers, and being unwitting sleeper agents for the Aboleth.
... Actually, in many ways, the Gillmen are worse, because not only do they have this annoying railroading "plot hook" built into every single character, but they miss the point of Aventi by having the Water Dependent racial weakness, which is one of the reasons why merfolk and tritons have traditionally been ignored.
