Not steep enough. You might have better luck near the airport. Hm... Looking over the Getting Out of the Water post by fnxrak it looks like his alternate spot is a bit different than the one I used for slow or tall vehicles. I generally beach south of the hangars, although sometimes it's difficult to get much momentum build up there. Maybe his spot works better.
When I brought a Barracks to LS I nabbed the one at Area 69, undrowned at Hampton Barns (blocked by a large rock on PS2), and beached east of Palomino Creek, but I'm not sure if it's the same spot as shown on fnxrak's map. I was using a beach recommended by GTA Phreak. I'll see if I can find some reference.