First thing I noticed was that the horizon is not level. It's not off by much...but it should always be level when water is in the background like this.
Also, you bisected her head with the horizon. I don't think it's all that bad in this case...but it's not good either.
I also agree that she is underexposed. I think the background has a nice exposure so to brighten her up, I'd suggest more fill light via flash or reflector.
Overall, it's still a fairly good photo. Her expression is good, which goes a long way.