[Edit: my compliments, mdk and I didn't give the 'down' thumb!]
There are thousands if not millions of so-called 'marinised' automotive engines installed in boats. Most likely the majority.
Actual 'thoroughbred' marine diesels are fairly rare, by comparison (Gardner and Bukh are two such examples)
The BMC 1500, which is virtually ubiquitous in many older inland waterways boats is not only a marinised automotive engine, but a diesel-ised petrol engine! (The cylinder-head having been replaced by a heavier one to increase the compression ratio and facilitate the necessary injectors, and the distributor having been removed and replaced by an injector pump).
The main thing involved with a marinisation is the cooling.
Instead of routing the normal coolant (which is the same) through a conventional radiator, it needs to go through another form of heat-exchanger which in turn is
cooled by river / sea water.
You must not simply run the saltwater through the engine. Automotive units are in no way at all suitable for direct cooling with saltwater. They would self-destruct rapidly. Thoroughbreds can deal with this well - they are built using marine metals - but even these last much longer if this system is used.
This can be by simply running it out through the hull and along a series of pipes fitted near the keel ('keel-cooler'), or into cooling tanks - internally formed into the skin of the hull and cooled via simple contact with the 'waves' through the vessel's skin. The 'Oakley 37' RNLI lifeboats - one of which I once completely rebuilt for the RNLI had enormous bilge-mounted exchangers with 'scoops' through the bottom fore and aft, supplying them with a large capacity of seawater. If the temperature needles began to creep up while idling but not underway, a short blip in forward or reverse was all that was needed to change the water in them for new and cold!
This would in most cases however best be addressed by using a standard form of marine exchanger (made for the marinisation process) and appropriate for any particular unit you have in mind. And fitted to the top of the engine itself.
The engine's own water pump sends the coolant around the primary loop and it is the same, enclosed coolant, with antifreeze, etc.
The secondary circuit is drawn from the river by a powerful marine lift-pump - such as a Jabsco - via a proper bronze intake seacock that will have a shut-off, for maintenance, and a water strainer, to filter out weed and other rubbish.
If you redirect this, external, water into the exhaust system after it emerges from the exchanger (so-called 'wet' exhaust) there is no need to have a hot type. Which are more hazardous, noisy, corrode easily and must be lagged. And are only necessary on air-cooled engines. (Or if you use keel coolers or skin tanks). Wet exhausts can use appropriate rubber exhaust hose. Which is easier to handle and is non corrosive.
Be sure to fit a properly made (or purchased) silencer of appropriate capacity that is intended for wet systems.
A water-trap must be included in the line and be placed lower than the exhaust outlet from the manifold. Preferably at a 'lowest' point. This trap (different from the silencer) needs to have enough capacity to contain all the water that is in the line 'downstream' of it. That is to say, between the trap and the thru-hull exhaust outlet. (So water does not run back into the engine when you stop it).
The water-injection point (where your 'hot' saltwater is injected into the exhaust for discharging through the exhaust system) should be a properly installed feature, welded - at a fine 'directional' angle - into the end-piece or 'spigot' of the manifold, just before the rubber exhaust hose.
If the outlet through the hull is a good bit lower than the manifold this is not quite so important but still highly advisable as it will have a considerable additional silencing effect and will protect against waves entering and reaching the engine.