There is nothing at all wrong with Barr aftermarket manifolds, where you can get a leak is the gasket between the manifold and the elbow, this is a known weakness in both this and the previous gen Mercruiser wet exhaust joint, even if the manifolds and elbows have not rusted through you can have a leak there. For one thing in salt water there is no way you will not find rust in a manifold or elbow used for 4 years. Anytime you find water in the oil the first thing you should do is remove the spark plugs, disable the ignition system and crank the engine over; if you get water shooting out of any of the cyls it could be the exhaust but it also could be a blown head gasket or cracked head. So if you do get water then, checking the exhaust is a good first step. If it does not leak then you have to move on to other sources. If you got no water out of the cyls but you have water in the oil, then you can also have a leaky intake gasket (would put water in a cyl and oil) or the area right under the thermostat housing in the intake (water passage between the 2 cyl heads) can rot out and this will put water right into the cam valley (this would put water in the oil but not cyls).
If your exhaust does not leak, but you have water in the oil, I'd do a compression test, this may tell you if you have a leaky head gasket, and then drain the cooling system and pressurize it with air. If it does not hold pressure, then its leaking somewhere the challenge is to find where.
I had water in the oil and in 2 cyls a few years ago. I tested the exhaust and it was not from that. While comp test was close to normal, checking for air bubbles in the cooling water revealed that combustion gas was getting into the cooling water, which means there is a breach in the head gasket and/or cracked cyl head. I took it apart and found both head gaskets blown right next to a water passage. I had the heads checked out and both had cracks from a previous overheat. I wound up buying a set of reman heads because the block was still good and putting it all back together with new (Barr) exhaust. The challenge here is in your diagnostics and you really want the water out as soon as possible, changing oil as often as needed to prevent cyl wall rusting and damage to bearings.
A quick test for exhaust gas getting into the cooling water is to replace the manifold feed hoses with clear plastic hose. Run the engine on the water hose till it warms up enough to open the thermostat. Once the 'stat opens, water will flow out of the block into the manifolds, if this water is aireated, than you may have either a blown HG or cracked head.
some pix of the teardown and re-assembly, last one is a vid of it up and running....
https://www.dropbox.com/s/tr23hyy6srwfnqv/4.3 manifold removal.JPG?dl=0
https://www.dropbox.com/s/k18krrafa34899f/4.3%20cleaned%20up%20block.JPG?dl=0
https://www.dropbox.com/s/8fhnjbtutfjejkm/4.3 reman cyl head 2.JPG?dl=0
https://www.dropbox.com/s/1yyytu26xgohers/installed%20reman%20cyl%20heads.JPG?dl=0
https://www.dropbox.com/s/55j6h4dd1u8fv0k/top end rebuild 4.3 test run.MOV?dl=0