When two young lovers get a motorboat and escape on a romantic weekend adventure, a fatal mistake leaves them stranded on a dark island outside of Stockholm for at least the night.

Hello,
the question is, then what happens ?
The reader (me) needs some proof that it's an horror movie.

Agreed with the above.

In addition though, the inciting incident is the "...fatal mistake..." but due to its generic description the specific nature of it is un clear. Best to specify what the event was, so it is clear how and why the main characters are forced into the situation they end up in.

Hope this helps.

As bamgomes said.? The genre is horror.? Well, what's so horrible about 2 lovers being stranded on an island overnight?? The threat to the young lovers needs to be identified.