What do sirens look like like?

Answer 1

Sirens are mythical creatures who were dangerous yet beautiful.
They are normally portrayed as "femme fatales" who captivated doomed sailors with their enchanted singing to shipwreck their vessels by luring them towards rocks.


To answer your question:
In early Greek mythology, they were portrayed as large birds with women's heads. They had feathers and scaly feet. As you would imagine, they were rather unattractive.
Sometime later however, sirens were mainly portrayed and described as beautiful nude, humanoid women.
Sometimes in the modern era they are described as being water-like manifestations of nude women, like a ghostly figure but composed of water. However they are most commonly, especially in classical literature and art, simply described as incredibly beautiful nude women with powerfully enchanting voices.


Sirens are not mermaids, but the two are often mixed up, especially in literature and film where the author or writer hasn't done enough research into the topic before writing.
Sirens are merciless killers who use their beauty and voice to lure people to their deaths.
Mermaids or merfolk are half-fish, half-humanoid figures and do not sing to lure people.


Further Reading:

• Sirens on Wikipedia
  

Answer 2

Sirens sometimes were just said to be lovely women, as a origin to mermaids.