My
Bukh 20 doesn't have a manifold drain and one on the block is just a drain. I think the tubes were just draining to a convenient container.
If I were you I'd take the front water injector off and clean out the
salt around this area. Drop the manifold off and remove the anode
plug then fire some high pressure water into the water jacket to remove the sediment.
I'd also check the
exhaust elbow for carbon buildup inside by unscrewing or unbolting at the manifold. These usually only last a few years. I just replaced mine because it had cracked and the buildup of carbon had reduced the hole to about 12mm.
Pleased to see yours looks in pretty much the same condition as mine.. A repaint would be nice.
After 23 years of
salt water there doesn't appear to be any
corrosion inside mine so I'm not so sure I'd bother with converting to fresh water.