Nothing to see here, move along.
This one's bad. I couldn't do its badness justice - we'd need to find Simon Cowell. It's hard to care about any of the characters, rendering most of the movie very dull, and if you're gonna do a love story, you should... well, *chemistry* would be nice, as would *having the characters actually get to know each other.* As it is, they...
--spoilers--
...don't until the daughter, having tried to get them together for the better part of an hour, (oh, did I mention she was the one who decided the dad needs romance? He was just fine being on his own, and worried about proving his point to the other academic muckety mucks - which also winds up lame in the end: his speech basically amounted to "the ancient Greeks were nice" and didn't prove anything, but the academics cheered anyway 'cause it's the end of the movie and they're supposed to.) gets her to take human form and help him with his research and they wind up translating and accidentally activating a love spell. This could have been handled well, but was instead used to allow the lazy writers to skip over anything like having them actually get to know and care about each other. The movie basically does nothing for the better part of its run - I had to fast foward through several tremendously dull parts and am not surprised that so many walked out on it - and then uses the love spell thing as a way of skipping to "happily ever after" without having to work for it.
--end spoilers--
So, forget this one. And I've heard the "it's just a kids' movie" defense used before, here and elsewhere, and it just doesn't ring true. ET is a kids' movie. Wall-E is a kids' movie. Toy Story, Ice Age, and Monsters, Inc. are kids movies. Avatar is a kids' show. Harry Potter and Animorphs are kids' books. We all know and have seen that kids' movies *can* tell a decent story and have likeable characters, and *frequently* do, to the point of some of them (such as the aforementioned ET) being seen by many people - not all of whom were kids when they first saw them - as the best movies of all time. So it's *not* unfair to have standards and expectations for a movie whose target audience is below your age. And since kids tend not to be known for their attention spans (something Harry Potter seems to be disproving, but hey)... having long periods of time in which nothing happens is *less* forgivable, not more.
There are many movies that are targeted at kids and manage to be excellent. This one is not excellent. This one is not *decent.* This one is worthless.
If you want your kids to see a good mermaid movie, they've already seen The Little Mermaid, and Splash has a few naughty words too many for your liking, I recommend Aquamarine.