They are virtually the same except for a couple of differences. A "Marinized" engine may have a cold water intake instead of a closed cooling system like you have in your car. If it does have a closed cooling system (radiator, hoses, etc) than it will require a heat exchanger so that the hot water in the system can be cooled.
Exhaust systems provide a certain amount of back pressure to the engine that helps to reduce valve burning and other problems. The exhaust system in a boat is designed to run on water that not only does that but acts as the baffle to reduce the noise. The cold water intake is either pumped through the engine and into the exhaust system or it is pumped through the heat exchanger and into the exhaust.
You may want to do some additional research regarding the thermostat and water pump and see if they're sufficient for the job.
The cooling system and the exhaust system are the major differences. Most automotive engines can be "marinized" without too much difficulty.
On a car engine, coolant is pumped through the engine block and then through a radiator, which is air cooled. On a marine engine, coolant is pumped through the block and then through a heat exchanger, which has raw (sea) water pumped alongside to cool the coolant. This is referred to as "Fresh water cooled" as no sea water flows through the engine block.
It might be simpler for you to just go to a straight "raw water cooled" engine... disconnect and remove the radiator, and attach a seawater inlet hose to the water pump. This will allow the engine to pump the ambient water through the engine to cool it. However, this must also be attached to the exhaust system... thus the difference in the exhaust manifolds as mentioned by the previous poster. Unlike a car, which has lots of air moving over the exhaust to keep it somewhat cool, a boat's exhaust plumbing is all enclosed inside the boat and must be cooled by water. Therefore, once the raw cooling water has been pumped through the engine, it must be dumped into the exhaust manifold.
Ok, so that's that. I assume you already have a transmission in the boat? That's a separate story.