It was probably your own damn fault for not making sure the clamp screws were 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 a 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 damage to the vital machined surfaces. Namely, rust. As soon an outboard is out of the water, remove the spark plugs and crank the motor to purge it of water. Squirt, pour or spit an ounce or so of oil into the cylinders and crank again.

Some experts suggest pouring 200 proof alcohol into the engine to absorb water, before draining it back out and introducing oil.

Then start the engine and idle it for a few minutes. Because your engine has been sitting, and presumably waterlogged cylinder walls, piston rings and precision roller bearings may already have rusted. Also, 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.

If you can't run the engine right away, pickle it. Fill the cylinder with oil.

To prevent it from ever  happening again, gnaw off one of your arms.  Either that, or drill a hole and bolt the motor to the transom.

For more marine engine stories:

  Marine Engine Digest Home Page