It was probably your own fault for not making sure the clamp screws were clamped down tight enough.
To make a long story short, the outboard motor hopped off the transom and sank in about eight feet of water.
You were able to estimate the submerged outboard's depth thanks to the length of the fuel hose buoyed by the floating fuel tank leading down to the submerged motor. So now your question is what to do before running the motor again?
Losing an outboard motor overboard is just like being thrown from a horse. You've got to climb right back on and ride, immediately. Only in the case of an outboard motor the reason isn't a matter of self-confidence, but instead damage to the vital machined surfaces. Namely, rust on the cylinder walls and piston rings. So if you can't run the engine right away, pickle it. Fill the cylinders with oil, or even leave the outboard motor submerged until you can fix it.
As soon an outboard is out of the water, remove the spark plugs and crank the motor to purge it of water. On a four-stroke, drain the crankcase. Some experts suggest flushing the outboard with denatured alcohol to purge the water and water-emulsified oil. Squirt, pour or spit an ounce or so of oil into the cylinders and crank again the engine to spread it out within.
Remove the carburetor and disassemble it. Flush every passage and jet with carburetor clean. On small outboard motors with integral tanks, or portable tanks, drain whatever's inside and take it to a recycling center. Do not dump it on the ground. Flush the tank with denatured alcohol and drain it all out. It is best to practice to pull the flywheel and flush the area where corrosion would otherwise take hold.
Start the engine and idle it for a few minutes to warm it up. On a four-stroke, after a few minutes change the crankcase oil. You will be surprised to see how much water is in the oil.
Worst case scenario, the waterlogged cylinder walls, piston rings and precision roller bearings may have rusted. Another possible problem, when an engine is running and ingests a great quantity of water, incompressible liquid may bend the connecting rods. It's not a pretty sight.
To prevent the dreaded submerged outboard motor syndrome drill a hole in the transom and bolt the motor to it.