i think, your problem is caused by the following:
1. you shouldn't be able to drop behind a cliff, or float up there so you can sit on the top of it anyway; unless it's meant to be accesable without gravity alterations. then your map making is poor and you should lower the height of the cliff edge that you can reach (without gravity alterations). because water always looks dodgy from a distance, especially if you've used 1 big square brush (of water) that sits inside several other brushes (the cliffs/mountains of which you spoke).
this is noticable mostly in the final boss in halflife, when you get shot up into the air, look at the boost pads, they are flush with the water texture and therefore start to have lines accross them where the two textures are "merging".
2. make the sky box directly connect to the top of the cliffs, so even with gravity alterations the player cannot possibly get behind the cliffs, or to any places where he/she can see your poor map making.
3. if you are viewing it from far away, then it really doesn't matter, because the player will never get into that position in a normal game. when mapping it is important to remember that what the player sees and what is actually there are often very different things. for instance, it is tempting to alter the x-axis of the top of your cliff brushes, but this might not be neccesary, as depending on where a player stands, the cliffs will look different anyway, and give the impression of variation even though they are plain flat cliff tops.
4. a screen shot of the problem would help us a lot. use www.imageshack.us (infinitely free hosting, max 750kb file size - support it today =D)
5. don't attack people here, even noobs, it' unbecoming and arrogant. that goes for everyone, even me (sorry coolfat lol
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