Let's begin this off by speaking out about how dumb of an idea it is to make movies out of board games, of all things. Board games that have literally no connection with such media whatsoever. Battleship being one of those games. The fact that Hollywood has to do these kind of these movies to make money is honestly kind of sad.
Ironically enough, the movie Battleship actually doesn't have a whole lot to do with the game it's "based" off of. The movie centers around an alien invasion on Earth, with the alien ships centering themselves in the sea. It just so happens that they decided to pick the time of the RIMPAC war exercises in Hawaii to show up.
Backtracking a little bit, we have our main character; Alex Hopper. A skilled person with unbelievably horrible decision-making skills. He's a Lieutenant in the Navy, though he's expected to be kicked out fairly soon. Then the invasion happens. Three destroyers are caught in the forcefield that the aliens put up to keep out interference. Plenty of people are killed in the initial attack; leaving him somehow in charge of the last functional destroyer. Thus begins the war for Earth. At sea.
As mentioned earlier, the film actually doesn't have a whole lot to do with the game Battleship. The only time they truly sneak in elements of the game is during one of the better sequences in the movie, when both sides have nonfunctioning radar, forcing them to play things a little differently (Battleship-like coordinates are included during this admittedly awesome, suspense-filled scene). However, that's just about where the similarities end. In one of the biggest crimes of the film, no one even says "You sunk my battleship!" (And it would've been awesome to hear Liam Neeson, who was in this film, say that.)
This film does have some good and bad things both going for it. It has some pretty thrilling action sequences at times, and the visual effects are pretty good. After a rather slow first 30 minutes or so, it's a pretty relentless fast-paced film after that. However, it is plagued by quite a poor script and mediocre acting from Taylor Kitsch and Rihanna (who seriously needs to get out of the acting business and stick to singing), though they and everyone else weren't helped by the poor script. There are some just flat out ridiculous moments (namely the introduction to Hopper), and the plot isn't exactly perfect either.
Overall, it's a fairly entertaining action film, even if the fact that it exists is somewhat ridiculous. Luckily, I don't think most would expect much from such a film, thus there won't be too many disappointed moviegoers. Knowing what to expect when going in will help this be a passable and occasionally quite fun popcorn flick; as that is pretty much what it is. Fans of simple action films and special effects will have a decent time. Otherwise, there's not much to get you interested in the first place.