I'm also horizonally challenged, especially if I'm not really taking my time.
The built-in electronic levels are great if you can activate them in the viewfinder of whatever camera you're using. Fantastic for improving the horizontal accuracy of your handheld shots.
When on a tripod, again, the electronic levels display on the LCD is wonderful. Test it, prior to relying on it, by shooting a known good construction level relative to your camera's level. I found most of my level-equipped Canon's are within 1/4 degree or less, one D800 is a little off, about 1/3 degree, but I now know by how much and can easily remember to compensate for it now. i don't think there's a way I can recalibrate it.
TRIPODS are helpful as they allow you to slow down your composition and look at the features. Calibrate a hot shoe level and your camera against a construction level and remember how to use it in the field.
I found most hot shoe levels are low precision but if you're really bad at getting level, they're helpful.
Obviously, distant water horizons are pretty darn level.
irregular waterlines can't be relied on.
You can use the water horizon to align an edge of your viewfinder to be parallel to it.
if you have reflections in water, of trees or poles or something, then the points of a reflected object should form a vertical line to use as a reference. E.G. The tip of a small tree and the reflection of the same point. Only works well if near the center of the frame. Using wide angle and some vertical tilt wrecks this method and you'd have to do some extra steps to get level. Wind will wreak havoc with this too.
Viewfinder gridlines are really useful. Activate them if you have them electronically in viewfinder or live-view. Sometimes a slightly off-level image will be a better composition than a level one so use your best judgement. It drives me nuts tho, when I see tilted water horizons or leaning buildings in peoples' photos.
Replaceable viewfinder screens with grids are useful too, adding one to my 5D2 really helped as you now have reference lines closer than viewfinder edges to work with and they can really aid as composition guidelines for those who like the "rule of thirds."
If you don't have any of those to work with then the AF points can be lined up for horizontal and vertical references.
However...
Some cameras (coff coff, D5100, coff) have some sort of problem where the viewfinder's AF points vs the final image coming off the camera are not parallel. Both of my D5100s are off by nearly 1 degree in the same direction! I didn't notice it for a while until I started shooting some water scenes. I wasn't using liveview and the on-screen gridlines as I was trying to conserve what was left of my battery for more shots. So I leveled the water horizon carefully to the outermost AF points. All those shots were consistently clockwise from level by about 0.8 degrees. ARGHH! I've yet to send them in to see if they can straighten them out, likely the reflex mirror is a bit tilted as everything else seems to be optically well aligned. I now have to try use them as follows: compose, level, then rotate body clockwise the amount of alignment error to the best of my ability to compensate - drives me nuts when I'm in a hurry.
In some cameras the sensor may be tilted relative to the body so you have to do some testing to see how yours behaves.
Best feature I found is Pentax K5 - it will rotate the sensor assembly +/- a degree or more to maintain the sensor level to its artificial horizon so you can concentrate on composition and the camera will make sure the shot comes out level. Only possibly with a sensor-shift body like the Pentax. I often wish I had the K5 for that feature alone.
And one last thing... PRACTICE.