Outboard depth in water

[Garrobo]

If you hear a boat motor nearby, grab ahold of something stationary.

 

[mfalco]

Or best bet get someone to act as boat safety cover to shield you by putting their boat between your marker and the oncoming vessel.

This is definately a good idea. Someone on the surface try to keep boats away will have a lot better chance than a flag, or luck.

The only time i've ever been scared while diving was when I heard an outboard when I was in 10'-15' water.

 

[Doc]

I think the real issue and answer already came out.

But to answer the OP question- there is absolutely no standard length.

You can buy "extension casings", frequently used by sailboats in the 18~25' range. They drop the prop waaaay down. You can stack them up to go even deeper.

Then the owner might just put it on his dinghy. There's your MythBusters version.

Here's what they look like...

 

[FFMDiver]

I didnt expect this level of detailed feedback, thank you very much

 

[DeepSeaExplorer]

I've also seen a boat that should have had a short shaft engine with a long one, causing the prop to be much deeper in the water than expected.

You shouldn't be diving in areas where it interferes with navigation. You can't simply put your dive flag up in a busy channel or marina. If you're going to work in those conditions, you need approval / coordination and a support vessel to keep other boats away.

I've done search dives in marinas and I always try to stay near pilings and docks, which is where most searching needs to take place anyway. If you need to surface, swim underwater to the nearest piling and come up on the inside, under the dock.