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Although it may not be the cause ... that you mentioned a fouled exhaust manifold ... so, include in your check: for a stuck exhaust valve due to a pin hole leak (between the cooling side and the gas side) in that manifold. If there is a pin hole leak in the exhaust manifold and water is now (or has been) 'back-flowing' into a cylinder it will surely 'rust' the valve stem on an exhaust valve and possibly cause it to 'stick' in the open position. You can check for a stuck valve stem by simply removing the valve cover, hand cranking the engine while watching for movement of the valve stems and rocker arms.
If a stuck valve stem, there are many ways to get it free again and without a 'rip-down' (penetrating oil on the sticking stem, etc.)

If an exhaust valve stem is stuck or not closing correctly, you will get 'white' exhaust due to the unburned fuel due to the insufficient or NO compression due the valve not closing completely/correctly or not at all.

If the manifold has developed 'slab rust' (BIG flakes of rust breaking loose from the manifold internals and which block the manifold water outlet, you can easily have cooling issues. Such rust-slabs usually will fall back to the bottom of the water chamber of the manifold when the flow is stopped. This may not be the cause of non-running engine, but may be the cause that initiated that failure - back flow of water into a cylinder. The way to quick check for internal leaks on an exhaust manifold is to attach a pressure water source to the inlet nipple, block the exit nipple, apply pressure, and either watch for water from 'gas side' or with a valve at the inlet water side close it, and see if the pressure applied 'holds constant'.

Also be aware that during a high humidity day during cool fall/autumn weather, that water vapor can condense into 'visible water aerosols' AT the outlet/overboard exhaust port of the boat ... the hotter the exhaust water the greater the 'white cloud' effect.

Most of all, do your checks and analysis for the problem in an ordered and logical progression - simple things which dont require 'disassembly' --- first!

From your quite brief description, I would be inclined towards the following:

1. blown head gasket or stuck exhaust valve, etc. (compression)
if blown head gasket 'between cylinders' and also involving the cooling passages to the head, etc. would include 'white' exhaust (steam).
2. fuel system problem (a routine standard-practice 'usually high probability' with diesels)
3. Blocked fuel injection elbow to the water lift muffler (but wouldnt yield low compression, just high 'back pressure' to ALL cylinders).
4. cracked engine block, or cracked or warped cylinder head.

As stated above ... with marine diesels, always check for blockage, etc. in the fuel delivery system - FIRST.

hope this helps. Good luck.
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