I think this can easily be achieved. One unknown is the mark itself - are you planning a big inflatable mark that’s need to combat the wind?
A quick look around this site (4 thrusters, 4 escs, battery, charger) you are at $1214 plus delivery.
You would need a some sort of floating pontoon and a water proof box.
With electronics, you are probably at $1500 to $1800 all up
You could do the “raspberry pi / pix hawk” or “raspberry pi / navio2“ option plus gps.
However for a fraction of the price possibly an option might be a bulk standard drone setup with flight controller and 4-in-1 esc. The gps (and a FPV camera!) would just hook Into the flight controller. You would be able to remotely confirm the mark was in position or move it in a wind shift. You would also have remote control or flight planning where the depending on distance, the mark took itself out to the point and came back when finished . You could also have remote FPV camera and barometric pressure. Could be a very smart and capable mark…
Complete setups with FC, ESC, camera, video transmitter frame, gps are under $140. You would also need a remote control. Just an idea…