20000 lumens monster canister light—WURKKOS Most Powerful Dive Light Yet

[CT Sean]

It's a worth method for trying. How much depth is appropriate? As my feeling earlier. The deeper water, the buoyancy greater.

Depth is irrelevant here. As long as the body doesn't deform under pressure it will displace the same amount of water at 1cm depth as it will at 10km.

If you want a quick and dirty test of submerged "weight", fill a container with water to the very edge. Put the flashlight in it. Weigh the water that spilled over the top. Subtract

 

[SouthernSharktoothDiver]

If you know the volume and weight, you can calculate density and solve for buoyancy. Alternatively, density can easily be measured by submerging the light in a tank with volume hash marks (like a huge measuring cup).