It is difficult...
Unless, as most say, really powered, it can not be done.
You can start in water with same depth as your foil - sometimes - sometimes not, if powered lightly for normal carving or waveriding...
But often board and foil will sink so not possible.
When sufficiently powered - you can tilt the board on its side, A LOT, as otherwise the wing will catch the bottom, and unless it is really low AR, it is crazy tough on the wing and fuselage - close to destruction.
If depth is "close" to mast length, you can usually start if okay powered, "flat", thus no risk if you touch the bottom.
Is it to save time you want to know if possible?
Or because it is too low all over?
It the latter, I would get a shorter mast instead, as you have no gain in a normal mast if it touches the bottom on touchdowns...
IMO starting with board flat in low water, is the safe way to avoid gear damage.
Peter