Hello,
Nice project!
You have to trust your plane’s behaviour in order to try it over the water 
We also used a flying wing (already available in our company) and tried to turn it into a VTOL.
We first tried to use only two front tilt motors but it would be too sensitive to the wind.
Recently we modified it to a tri-copter configuration (using the same front tilting motors) and all the pixhawk parametrization was based on the information available in the Aticof’s project:
We ran into several difficulties during this modification but now succeeded (at least with no crash) in a controllable vertical flight (no adjustments to PIDs were still performed):
We now have to try to correct the vibrations caused by the back motor as you can see/hear in the video (we still don’t know if it can be corrected by reducing the pitch PIDs or if we have to increase the rigidity of the rear part of the foam structure).
We also want to use AUTOTUNE to adjust the PIDs but we will not have enough battery to fly for a long period of time (unfortunately it has a 5800mAh 4S 60C battery and only flies for about 3min with wind 
).
Does anyone knows if we can “pause” the autotune, land the aircraft, change the battery, take-off again and continue the autotune?