I wasn't expecting much from this film at all, and gave it the "let's watch the first 10 minutes, just for a laugh" line to my wife.
We ended up watching the whole film and really enjoyed it.
Yes, the plot is farfetched and ridiculous but it's well acted, well paced, tense at all the right times, and (somehow) managed to find the right balance of seriousness.
It ain't Jaws, but it'll do.