Contemplating ideas for my long gestating longer story. There's a "weird alien" in the story that is not really working for me anymore so I want to replace them with something more ... relatable. The idea I have in my head is an amphibious hominid. Not merfolk with fish tails. I want something that feels more mammalian and, more importantly, is amphibious, i.e. can function both on land and in water. Traditional merfolk can't do that without magical transformations. So maybe more like a pinniped (seals, sea lions, walruses) x human hybrid. Basically human but with adaptations allowing them to stay longer and be faster in the water than normal humans. What might that be? Well, I am still thinking that through but so far I've got.
Either hairless or covered with short, fine hair that lies flat to the skin (let's face it, Ariel's long red hair looks great but has to be hell for her hydrodynamics).
Sleek, fairly muscular body, esp. hips and downwards, for better hydrodynamics and more powerful swimming
Either a greater lung capacity or more efficient distribution system for storing and distributing O2 in their body, enabling them to spend maybe 15-30 minutes submerged*.
Eyes adapted to seeing in both air and water, probably with compromises in both.
Small ears flat to the head. Flat or small nose.
Feet that are flipper-like, making them better in the water than humans but less efficient at running on land
Male genitalia and female breasts minimal, maybe retractable for the men. Again, for hydrodynamics.
Generally naked (it's a tropical environment anyhow) since clothing would be a liability in the water.
Technology somewhat limited due to living partly in water, maybe a very sophisticated neolithic level.
Footnote:
*By comparison, humans have done this but (a) with pre-oxygenation (hyperventilating pure O2) and (b) they were static, i.e. just sitting on the bottom of a pool. Without pre-oxygenation and swimming, the record is something like 11 minutes and most people can do a minute and a half tops. These pinnipeople would do half an hour routinely and be actively swimming while doing it. Static and with preparation, they could probably do an hour or more.
As for the adaptations, there are islanders capable of long submersions who have larger spleens to produce more red blood cells. Aquatic mammals often have more myoglobin, a muscle cousin of haemoglobin, than land mammals, too, which stores more oxygen and is less affected by the acidity that builds up during long immersions.