The advantage to closed cooling is that you have pure and clean antifreeze actually touching the block and the majority of the cooling system.
However, the sea water (salt) has to enter somewhere to do the cooling (of the antifreeze) and exit somewhere out the back of the boat. Hence, these two places are where you can and will have salt water issues if you do not flush. I guess its a matter of personal preference if you do not want to flush, and would rather replace parts after a while.
As for the 525s, the CMIs are notorious for being delicate, when it comes to salt water. For many (not all) boats/engines, its not if your headers leak, its when. If you have/get a leaky header, its not going to be pretty or cheap.
As for the 496s, (I have them in my boat) with cast iron exhaust, you have a little bit more robustness to the exhaust, but the cast iron is actually more susceptible to salt/rust corrosion than the SS of the CMI headers. Even worse is that the salt water actually drains off from the majority of the exhaust system, just by gravity. This leaves the metal that has been exposed to salt water, with the salt deposits, sitting on the metal, exposed to AIR. This is basically a petri dish encouraging salt/rust corrosion.
As for the heat exchanger, virtually the same on the both the 525 and 496, there are many tiny tubes inside the heat exchanger that the sea/salt water run through. If/when these tubes become salt/rust corroded, the cooling capabilty of your closed cooling system is going to diminish. Hence, you are going to run hotter. Eventually, the salt/rust corrosion can and will eat its way between the passages of the heat exchanger and will allow AF to enter the sea water side, and the worst... sea water to enter the AF side. Now, the benefits of your fresh water cooling system are out the window.
So, bottom line, YES, you do need to flush a boat used in salt water if you have a closed cooling system. You don't need to flush it as long as with a open cooling system, as there is no thermostat in the sea water side of the closed cooling system. However, you must get the salt out.
(BTW, it you use the little air pump on the 496 motors, to push out the salt water, you are pushing out the water, and replacing it with air, leaving the salt deposits to sit on the interior surfaces of the cooling system. Whalaaa... petri dish.)