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.