A small 2 lb wight with a ribbon works just fine, just set it on sand and you are good to go.
@uncfnp is correct. A few years ago, dive ops asked volunteers to mark Lionfish sightinings by tying a 3 foot ribbon of survey tape to a piece of dead coral, sporting a cork on the end, to give STINAPA staff a visual of where the critter was seen.
That was a few years ago and that practice has since gone away. I can comment that in May of this year, we found 2 pieces of these legacy ribbons, one of them still had a cork... of course, we collected and removed the trash.
A convenient way to work the Reef is to head towards the buoy. Once you are in the water and cleared entry challenges, take a compass heading towards the Reef buoy and stay the course. Once you get there, choose which way or direction you want to go and you won't need to set a marker. It's easy to miss the bouy at times, the light, current and other things one can get distracted with, but you won't be too far and it your buddy is looking for the same, you will find it.
When doing night dives, I'll tie a small battery tank light to the buoy line if we can quickly find it... don't waste too much time looking either. Or just set a 2 lb weight on the sand with a small battery powered tank light, leave it bobbing 3 feet from the bottom. Set the weight on sand and you are good to go.
One last tip for night dives. Get a couple of strobes. We leave them flashing on the shoreline. They have helped us many times over and over. The ones we have are not waterproof. They can get wet, but not submerged. These are the ones we've got:
https://www.amazon.com/UST-See-Me-W...85186&sr=8-12&keywords=emergency+strobe+light
We've gone night diving in Tolo, Jefferson D, Karpata, Tori , Red Slaves, and come up to a pitch black sky and shorelines... no light what so ever to help orient if we are near the truck or not. Had to try hit truck tail lights with a flashlight to see if the vehicle was parked in an area or not. Solution found with these strobes.
Enjoy your trip.