They won't say that and it's probably ok for a little while, but those hi-power transducers can get hot and burn out if your not careful. Why don't you email Lowrance and ask just to satisfy all our curiosity.
We have had our LCX- 16 for over a year. Have run it many times out of the water with no problems. We have had it on and using the simulator mode for over a hour at a time. It's one of the best ways to learn how to use your unit. I wouldn't worry about using it out of the water.