Detroit Diesel Starter set-up

The better marine solenoids will have 2 wires to enable them and not use a frame ground.
Your fault example is a main reason for this.
You may consider upgrading these in your future.

Also, pending the install requirements, the block is NOT tied to battery negative but floating.
Starter, alternator, shut-down, oil and temp senders will have their own negative leads.
Please check this out also; where does that negative battery lead (not bonding straps) go on the engine??

Interesting.
So there is No negative strap on the starter
or
a negative strap from the starter to the block.
or
a simple self grounding starter like on a typical Chevy car engine.

Next,
When was the last time these 53s ran?
Have they been turned over by hand and bar or spun with the starters?

You are correct and IMO the preferred setup.
Our new poster stated to the block.

Lots of the big Delco starters that were used (not all), were ground isolated. The negative cable had to connect to the starter.
When block ground was desired, an extra strap was from the starter to the engine block or a terminal block.
I have worked on Cats (same starters) with this same install set-up.

On my 32v Bertram, my 12V71s are floating and not tied to battery negative. Alternators have their own negative post also.
On the Hatt I work on, OMG, yellow and green 4/0 cables are all over the blocks, starters and DCv breaker boxes. Nothing floating there.

Hello mjkrawczy
You spinning/running yet?

You running yet? Got the water skis out?
You go first..

Don't mess up the ring gear with a pry bar.
I don't believe there was a service port or turning tool on the 53s.
Use the bolt in front of the engine. The big (15/16"?) crankshaft hex head.
The manual will say not to turn to the left and loosen the bolt.

After leaning on a long pull bar for a few seconds, reality may set in.
The design of the Detroit 2-stroke in a marine environment, not running for a long time, usually does not work out to well.
Plus, no telling what issues it had when the previous, previous (previous?) owners walked off the boat.
If the piston is set in any one lung, even if you could soak it and free it, it will still be damaged and dangerous to run.
This applies to the other engine that turns also.

Next hard question: How are you going to get those engines out of the boat?
Answer; In pieces, out the back door.