Sorry about that.
There's no magical way.You gonna have to drag it back in place and measure.
Maybe measuring first,drop a lighter anchor with bouy just beside the real spot,drop the 150 pounds bouy anchor as close as you can of the right spot and finetune position afterward.
Tie a temporary rope 6' under to move anchor around.Finetuning it in the water is gonna be at least a 2 guy jobs with a couple pfds or lying on tube.
Getting it back close to original spot could be done from the platform.
Good luck!