the 4.3s use a smaller diameter filter than the 5.0/5.7s
you could use a Sierra marine filter or an auto application for that engine like a Purolator or Wix...
The Merc and Sierra filters have gotten much smaller over the years, they are easier to install in a tight installation but not sure if they filter as well.
I used both Sierra & Merc over the years, no difference that I could see, or notice.
I have a remote oil filter mount for the 4.3 that I bought from Michigan Motorz. My only hesitancy in installing it is that some say you have to replace the bypass valve with a higher pressure one (30 psi vs 11 psi) some (including the guys at MM) say not necessary. On the 4.3 changing this thing is not as easy as on the V8s because the V8 uses a removable adapter to hold the bypass, the V6 does not (maybe because the diameter of the pad is smaller). The bypass is installed right into a passage in the oil filter mount and with the engine in the boat would be really tough to get at. So not sure I'm installing it this fall. Gotta think about it some more.
It sure would be nice not to have to pull out the rear seats to do an oil change like I've done for 20 years, though.
Wonder if Merc's remote oil filter kits mention this? I saw the parts that go into them, and never saw a different bypass valve.