This is not the kind of movie that will keep you guessing. There may be a few little surprises en route to the climax, but there is never any real question of where it is going. Whether or not you like this movie will most likely come back to the technical aspects, the acting, the direction, and the music. The story, screenplay, setting are all pretty much non issues, as the lead performances will have to captivate you, and the use of music will have to manipulate you.
Ashton Kutcher makes his return to the big screen for the first time since the abysmal A Lot Like Love. I am unconvinced of him as a leading man, but I also am willing to give him a shot. The Guardian strikes me as his first attempt at a big dramatic film, and for the most part he's successful. His biggest drawback is his voice and mannerisms which seem to be fully entrenched in the comedic realm. Pairing him with Costner is a good choice, giving the younger thespian a bit of his own star-power, and, hopefully, giving him some advice while on the set. The scenes they share together are among the most effective of the piece.
The rest of the supporting cast is effective, especially Sela Ward, in her limited screentime as Costner's long suffering wife. Melissa Sagemiller is also pretty good, emphasis on pretty, as Kutcher's love interest. Then there is the rest of the swim class. All have distinct personalities and all seem to fit in well.
Andrew Davis injects some nice visual flair, particularly in the stormy seas. They have a nicely visceral feel to them. The music, from Trevor Rabin, deserves mention, not because I particularly liked it, but it effectively plays your emotions for just the right dramatic effect. This is far from the first movie to do this, and it will be nowhere near the last, but this one stuck in my mind.
Bottomline. Not as bad as it could have been, not as good as it could have been. It is effective in what it sets out to do. It features some decent acting, and is generally good entertainment. It was a decent test of Kutcher's potential, but I am going to be optimistic and say his potential has not yet been reached.