What is meant by Dry sump? I hear alot about #6 Dry Sump drives. What is the deal? Thanks

I'm not drive expert, but I believe the #6 is the strongest drive Merc makes. It's used primarily in racing.
With a "dry sump" oil system the drive oil is stored in a separate catch tank in the hull. An oil pump pulls lube from the tank and circulates it through the drive where it's collected in a low area and pumped back to the tank.
Don't confuse a Bravo drive with an external oil tank for a dry sump system. The Bravo has the "oil reservoir" just to keep the drive full of oil.

Advantage?
With basically no oil in the drive the gears can be spun faster, with less drag. Also the pressurized oiling supplies better coverage to the gears and bearings than being totally submersed. With gears spinning in oil they can cause the oil to whip into foam or create an air pocket around the gear keeping the oil away.
Another advantage is that the oil tank can hold far more oil than just filling the drive housing. It also helps keeps the oil cooler.

Some racing engines use a dry sump oil system too. Same idea, no oil is stored in the pan. It all goes to a catch tank. With no oil in the pan the crank shaft can spin faster without oil splashing around.

pretty much, except the 6 doesnt have an external tank, but other wise on the money

If the six doesn't have an external tank than how does it work?

For lack of a better description the "dry sump" is actually a "wet sump"... but instead of the entire drive being filled to the top with gear lube, only the lower few inches are filled with fluid. The propellor shaft has a "gear/rotor" pump splined to it much like an automatic transmission in a car. So now the fluid can be circulated through nozzles to spray the lube on the gear faces and there is a lot less drag than having all the components "sloshing" waist deep in the lube.

OK, like I said "I'm no expert".
When I think of a dry sump system I think of having an external oil tank. I thought, assumed, that's how the drive was set up too.

How the drive is described would lead anyone to think that way Iggy. But there isn't a seperate tank. I think Merc just couldn't think of any other way of describing the difference between the #6 drives that are filled full of lube and the #6 that has a pump and only a few quarts of lube in the lower portion.

One of the reasons for using, dry sump systems on a race car is to keep everything lubricated at all time. On a "ordinary" system the g force could drive the oil away from the pump.

True. Otherwise the pan would need baffles and trap doors to keep the oil at the sump.
I had a '66 Shelby GT350, as I rounded a fast right hand curve I lost oil pressure momentarily. Spun to rods and one main bearing.
If the engine still had the original aluminum deep sump racing pan it would have been ok. But the stock 289 steel pan was on insted.

Yes.... you are correct... but another important reason for using a dry sump system on a high performance car OR BOAT, is the collected oil from the sump is pumped into a funnel shaped tank at an angle so the the oil "spins" along the outer periphery seperating the air from the oil before it flows down into the tank and is pumped back into the engine. That way the oil isn't foamy and lubes better...