Doc and Bob are correct with the answer for your question.
The main reasons why the bow digs in.
1. Too heavy on the bow
2. Not enough Bottom rake
3. Not a smooth transition from rake to bottom
4. Not enough power to get the boat to plane
5. Not enough bottom surface
6. The bottom may have a curve upward
7. Engine to for forward
With the airboats, the thrust that makes the boat move is located well above the bottom of the boat, connected by the engine stand. Basicly, no matter how the engine is position it will still have down force on the bow. The only real advantage of tilting the engine is if you tilt it so that direction of air blowing off of the props is in a downward direction. This is only to aid in getting the boat on plane, to put a little extra force on the bow, and it helps in making the boat slide around turns.
With the thrust point so high, it acts as a lever that multiplys the force on the bow.