Vaysa, it's rather strange because I worked quite a lot on the FM of this plane , in order to enable it to take off with maximal charge (torpedo or M bomb).
I have checked once again this morning on 4.122. The problem you have may be due to not following the procedure needed to have such seaplanes take off:
1) raise the throttle to 40%.
2) WAIT till the wake becomes really wide and the plane begins to move up to a speed 11-12 km/h
3) apply right aileron very strongly and push progressively the throttle to 70% - You must get a straight course. As soon as your plane is on a straight course, speed will build up and you can release the pressure on the right aileron
4) push throttle forward (no need to get even at max throttle, even 80% or 90% is enough to get over 100kmh)
5) at 100kmh begin to pull your stick and your plane takes off smoothly, even at full charge