I’ll try and explain how I used to use a floating spotter board …..good luck understanding it without a pic tho lol ……
I used a 2 piece plank , with a short middle length which bolted the 2 main planks together forming 1 long one …..
Under the formed plank I had a dozen curtain ring type things …..
I’d throw out a weight on a rope , and feed the plank down the rope via the rings and into the water ….
a few inches from the start of the plank I had a 20 mm hole drilled …..a long rod of reo slides thru this hole and into the riverbed …this holds the plank/board stable at the riverbank end ……now comes the important part ….
Slide a 1 inch pipe down over the reo …..a pipe 600mm long should do … this can push the plank down into the water , and use the rope to pull down the far end ….tie the rope off above the pipe …….this will set the depth at both ends as needed ….
I’m sure you will follow the above easy enough ………good luck