I went to see this today after convincing some friends that Mamma Mia would not be a wise choice for a Saturday evening's entertainment, and managed to get them to have their first taste of independent cinema, with Donkey Punch being shown in one of the cinemas in my area - it is limited syndicated release.
I'd say it was "good". It enters the territory of terror at sea and pulls it off well for a survival horror, especially a British one. The dialogue is sometimes laughable, in a bad way, literally, some of the lines were so badly constructed that the audience actually laughed. There are some fairly decent deaths for a low-budget feature, but a completely unspectacular "final" death, which really just spat on an entertaining climax. Character development was pretty confusing in places and it was so-so acted, but it had its charm and an element of forgiveness for being an independent film that actually made it into cinemas, even though it was a limited release. ANyway, I really have to go, this laptop's battery expires in about 10 seconds.