Yanmar 20 hp been under water recently

Hi,

Just bought a sailboat from the late seventies that recently sank for a day or two before being raised. I assume the Yanmar is from the same period.
This is a not an expensive boat, and I hope I'm able to use the engine without costly repairs. What damages should I expect? Any advice on what to do in what order? Thanks in advance.

Not too much problem starting a drowned engine, providing it is still 'wet'.
As soon as up, dismantle and clean starter and alternator, fresh-water wash all other electric connectors and spray, remove injectors and turn engine over to get out water, change oil and filter.
Let engine crank with decompression levers 'down' and injectors out until no more water coming out and you see fresh oil at the rocker arms.
Then install injectors and start.
let the engine run at operating time for an hour, change oil and filter.
restart and let run for a couple of more hours at light load and check oil again.
However if engine has dried up, it may be quite a different story!

I can buy a boat with a yanmar 2 cylinder 20hp that has been in dry dock for over a year.
The engine used to run fine, now it won't start.

What is potentially wrong with it?
Can it be fixed?

Bleed out the fuel system.
Loosen the nut on the injectors (pressure lines), crank the engine until you see clean fuel squirting, tighten the nuts while you still crank the engine.
Most likely a change of fuel filters and some new diesel may be needed, but you should be able to get it started 'as is'.