The Cast Show Off: Indiana Evans is a singer and sings the opening theme in the third season, as well as being shown singing at Rikki's club.
Dawson Casting: Averted in the first two seasons as the three leading actresses were still in high school, but the gap between Seasons 2 and 3 left them playing high school seniors while they were 20/21 in the final season. Luke Mitchell is a straight example, being about 24/25 and playing a high school student.
The Merch: CDs and books have become available in Australia; Kate Alexa's made herself famous by the CD. In the US, dolls of the 3 mermaids were available for a short time.
In the US, the last half of Season 2 and the entire third season were held up substantially. The show ended half-way through Season 2 during re-airings in 2008. When TeenNick began re-airing the series in 2011, the remaining episodes still weren't being aired - instead cycling through the same aired episodes again and again. Finally averted on January 16, 2012, where the final thirteen episodes of Season 2 were aired in a TeenNick marathon leading into the US premiere of Season 3. (3 Years after the UK aired the 3rd season.)
The rest of the world, including its native Australia, didn't get the 3rd season until 2010, whereas the UK got it first in 2009.
Post-Script Season: Only two seasons were planned. But the sheer popularity of the show led to a third being commissioned, as well as two spin-offs. As a result, Claire Holt was already committed to a film in America when the third season was greenlit - resulting in Emma getting Put on a Bus and replaced with Bella.
American viewers watching on either The CW and/or Netflix may be surprised to see Claire Holt and Phoebe Tonkin, knowing them more for their recurring roles on The Vampire Diaries and their main roles in its spinoff, The Originals.