Unlevel horizons especially in water related shots are very distracting to the viewer, unless it's intentional.
I find a ball mount indispensable and helps tremendously.
If using a zoom lens try zooming in at max and getting the level right first then go back to your prefered FL.
The quality of the mount won't matter IMO for your levels but as I say a ball head is great.
I've also seen a small spirit level that attaches to the flash Hot Shoe which sounds like a great idea.
If all else fails, run to Photoshop.
