Since I'm on the topic of mermaids, I figured I'd do movies in addition to the books I have already listed. These are in no specific order, but all are great mermaid movies or movies that have mermaids in them.
1. Splash (1984) - always a classic mermaid/human romance
2. The Little Mermaid (1989) - in my opinion, one of THEE best Disney movies ever made
3. Pirates of the Caribbean: On Stranger Tides (2011) - though not a very good movie, in my opinion, the mermaids were pretty awesome.
4. Aquamarine (2006) - though she isn't a mermaid 75% of the time, it's a cute movie based on an equally cute book
5. Magic Island (1995) - I don't know about anyone else, but I love Lily the mermaid & her tail is gorgeous!
6. The 13th Year (1999) - One of the only mermaid movies that is actually about a merboy. Gotta love it.
7. Peter Pan (1953) - who could forget the epic scene with the Mermaid Lagoon & the line "we were only trying to drown her"?
8. Hook (1991) - I have re-watched the amazing mermaid scene in this movie over a hundred times. They are gorgeous.
9. Harry Potter & the Goblet of Fire (2005) - Okay, well they might not be the prettiest mermaids but they're just so creepy that they're kinda cute.
10. Mermaids (1990) - So it might not have any "real" mermaids in it, but Cher's mermaid costume is amazing!