The "if" thing is a problem. Early primates weren't sea creatures. But I guess we can imagine sea-monkeys that aren't just brine shrimp. They're, like, monkeys who live on the coastline and swim a lot.
Long limbs and a distinct big head are a disadvantage for a swimming creature. They interfere with streamlining, so a swimming primate will be slower than the competition.
Those are also a problem in terms of retaining heat, but we can get around that by restricting the mermonkeys to warm tropical areas.
And there are the breathing problems, with the nostrils being pointed down. Each time it surfaces, the poor monkey has to lift its whole head out of the water and throw its head back.
What evolutionary traits made humans a success? Intelligence and tool use. Those benefits outweighed the drawbacks of the human form.
I don't think that would happen with the sea monkeys. Making fire won't work underwater. Banging rocks together to make tools, not that easy underwater. The intelligence and tool use won't help as much underwater, so the sea monkeys would be outcompeted and die off.
