Leaving it as is makes for a "one-look wonder". It needs something more, some mood perhaps. It could also use some shape, as the photograph is very flat.
The darkening I agree with, but not the severe crop. I see Kevin's point in making it less "obvious" or "direct", but without the horizon, it would appear more like a photographic mistake or oversight. A 16:9 would work provided the horizon is still there.
Adding a graduated mask to the "foreground" and background seems to help. But, as it is, it just doesn't do it for me. All these "additions" seem likely to make a 6 into an 7 when, perhaps, you're looking for an 8 or 9.
In the samples below, done in LR, the first is with grad masks only and a an adjustment bush along the "background" water. The second is the same. lightened by 0.5 in Exposure.
