Without knowing the Vintage or Make of your engine/ drive,..........
I Know you'll find a Block Drain, down low on each side of the Block,......
There Should be 1 in each exhaust Manifold,........
And,.....
Don't forget to Pull the Big hose from the Engines Waterpump..........
Mine has 5. Lower block just behind front engine mounts on both sides and bottom of both exhaust manifolds and bottom of large hose to the circulating pump. Some have more or less. What year,what 4.3 and what cooling system? Mercruiser says to put the plugs back in to keep rust off the threads in the block. Watch out for O-rings on the plugs. Easy to lose.