Re: 1ST boat, need help to get it in the water.
This might be a 275 drive.
Good suggestions for the sea water pump impeller, checking exhaust risers, fuel filter, timing, bellows, ujoints, etc.
You should replace the water neck fitting when you have the transmission off to do the bellows and u-joints. This is a wear item that needs to be replaced every several years.
These corrode and will cause a suction leak.
Look at the ring anode and the anode on the transom shield. They might need to be changed too.
Ditto Don S. No need to pull the whole drive, just the top gear box [aka transmission] and there is no engine alignment required once after initial engine installation on this type of drive.
Two orings for the trans and one each for dip stick and drain plug, and a new gasket for the oil fill plug.
That's four orings and one gasket.
While the drive oil is drained do a pressure and vacuum leak down test. The shift unit piston seal probably needs to be changed. If it is black or brown it's an old one.
The 280 s/p drive uses engine oil. 30W if in cold water and some use 40W if in warm Florida water.
Checking the PDS bearings is almost impossible without removing the PDS since this connects to the engine. You will be able to tell if they are extremely bad, but not if border line bad!
Best to assume that they need replacement and bite the bullet. If the rear bearing goes, you might have some pretty good damage here.
PDS is #65 here.
When doing timing, look at your OEM timing specs and check the advance at 3,000 rpm to make sure it's advancing the right amount. IOW, check more than just initial timing advance.