Both are closed cooled so the blocks are not exposed to raw water (salt) and have coolant circulating. The exhaust manifolds, oil cooler, power steering cooler, and heat exhanger are cooled by sea water so you still need to routinely flush when running in the salt.
The 496's stock exhaust are cast iron or aluminum (depending on the year) and resist corrosion better than the 525's stainless sweeper headers. The 525 headers require more frequent inspection as a result to check for leaks.
The 496's do have a quick drain system and the 525's do not. That helps drain the raw water side of the system for extending boating in cooler climates.