Detroit Diesel Starter set-up

Oh Sheet,
I felt it would come to this but my fingers were crossed for you.

The Detroit injector hole thru the head is pretty small. A faster path to look at the lungs is to pull the blower off.
You will be doing that anyway to clean it up. There is no fancy gear timing here.
Remove the blower drive shaft from the rear, undo the fuel hoses, hold down bolts, back gland clamps, back oil connect, slide it forward and up by the front.
Below is the air box and views of the pistons thru the liner ports.

Unless there was a water line issue (sank) there should not normally be water in the lungs.
Wet and damp air usually comes up the exhaust and rust up the valves and lungs.

You are going to pull the heads soon also. At least the blower will be out of the way when you reach over the motor to get to the far head.

Stuff is heavy with lots of edges and corners that like to smash fingers.
Be careful.

Is this a wood or plastic boat?

Yep, Side covers. Was not sure you would have the room to get at them.
But, You witnessed the damage.

Do you have room to drop the pan? Planning on an in-frame rebuild?

Those are one piece crank seals on the end. Your not going to be able to replace them during an in-frame.
This also means one main cap at a time comes off as to not damage these crank seals.

Remember, these are wet liners, drain the coolant.
When you get the heads and sump pan off, unbolt the rod caps, as you turn the crank, the pistons with liners will come up for you to grab and pull aweigh. Watch out for the crank throw coming back around that the other rod studs to not strike the rod journal surfaces on the upstroke. A 1x1 block can be placed between the rod journal and rod to push it up further for the (2?) near TDC pistons to get pushed up further. A third hand helps here.
It is the rods, pending the kit, maybe the pins you are going to re-use.

Now, time to get realistic; rebuilding a 53 may cost more than the value of this wood boat.
AND, chances are, you will be rebuilding the other engine anyway.

Add some Marvel Mystery Oil to each cylinder for a day before trying to rotate it again.

uh?