Interesting. Well, you know that the exit triggers span multiple y values, but it only matters if there's ground for the lemmings to land on. This is not true for water, where the lemmings will die regardless of surrounding terrain. So while the exit trigger's region may not be completely overlapped by the water, there's no ground for the lemmings to land on to make an exit, and they'll always hit the water first.
Timballisto: I've uploaded screenshots for your 3rd and 4th pack. One note: The method I normally use for piecing together screenshots does not work on Level 4 of your 4th pack (the really long line). Whenever there's no context and very little contrast in pixels either horizontally or vertically, it can take a really long time to get an accurate screenshot. On the other "line" levels, I've been lucky that they've been on something like the crystal set, where there's at least some variation in the pixels. The pipes make it a bit more annoying, although it should be possible, but it's late, and I'm tired A0;X_X
We're not reviewing that particular pack next anyway, so it doesn't really matter, but for the time being I just put up a screenshot of the level preview, which should give a basic idea of the level (although the picture is very very small.)