My two biggest concerns were the language and the behavior of John Candy's character. The Common Sense review by Heather Boerner says that there is "very little" in the way of language concerns. However, if you read below in the comments section, you will see that "britton26" correctly states that they say "g d and s o b and 1 use of the s word". If your kids are parrots, you might be concerned about that.
Heather Boerner then accurately describes John Candy's character thusly: "Freddy drinks constantly and gets Allen drunk after a wedding. Freddy smokes cigarettes while playing racquetball and smokes cigars in other scenes." I was able to explain to my children that the comedy of John Candy smoking and drinking while playing racquetball was because it was SOOOO unhealthy. It makes this point rather well, actually, as he is obviously overweight and not able to play racquetball as a result. If you want, you could even mention that this actor died as a result of being overweight (I chose to not go that far…). What I was not able to explain was why John Candy drops coins to look up women's skirts...I just said, "He's silly..."
As far as the message goes, I do take issue with Heather Boerner. She claims that Tom Hanks likes Daryl Hannah’s mermaid because "she's pretty and she's hot for him". Of course, it’s true that Tom Hanks does like her because she's beautiful, but is also clear that he has a vague memory of meeting her when he was a little boy, which is a sort of basic fairy tale device ("Sleeping Beauty") but also another way of describing what we would now call "soul-mates".
The message that is most useful for younger viewers, I think, is that Darryl Hannah's mermaid spends most of the movie afraid to tell Tom Hanks the truth about herself. This is often a problem in relationships: one person feels that the other won't accept him/her, so they cover something up (“I’m really a mermaid/cheerleader/jew/nerd/virgin/whatever!!!). If teens can learn to be honest with each other early in a relationship, so much the better.
Given that it is a fairy tale, of course, once Hanks learns the truth they go off to live happily ever after, which might or might not happen in real life, but at least the parties concerned would be living their own truth. As far as the “nudity” is concerned, I thought it was wonderfully appropriate. NONE of the “nudity” is sexual. It is all because, hey, she’s a mermaid and she doesn’t have clothes. The one shot of Tom Hanks covering his wee-wee is purely comic.