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.
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.
No Export for You: 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.