If it's leaking externally, it may be leaking internally too, but you can't see that happening.

You could pull the plugs on that side and turn the engine over to see if any water comes out and also look for water on the plugs.

A  machine shop planing job on the manifold and riser would be the second best solution.

First best but pricey  Not knowing how many hours on run time you've had in brackish water, you might at least think a little bit about maybe getting  new manifolds and risers depending on how ugly the internal water passages look

At least I hope you're not leaking water there 

If it's coming from one of the flanges with the red lines pointing at it, you've got problems.

The only thing that's supposed to go through those joints is exhaust gases

Having a lot of water dripping could be a few things.

Your manifolds are only 4 years old, so they're probably good and not leaking internally, but one could be bad

That would leave some other choices like maybe a blown head gasket letting water into a cylinder or maybe a cracked head or block letting water into a cylinder..

I'd start with pulling the plugs like mentioned earlier in the thread and look for water on a plug or plugs, then crank the engine over with the plugs out and see if water comes shooting out of any spark plug holes.

That should tell you something

While the plugs are out, a compression test would give you some info also

Pressurizing the engine cooling system with the hoses to the manifolds blocked off,  to let you know if the engine has a leak or not is also relatively easy

I haven't done anything yet except examine the manifold to riser gasket and all sides of the manifold. The water is almost certainly coming from the flanges as pictured above by Ken. I haven't pulled plugs yet, but am going to be really surprised if there is water in a cylinder. Boat runs great, doesn't get hot, and no excessive steam in exhaust. I'm guessing (hoping now) at this point that there is an inner casting that is has failed like you suggested and the water is escaping from the manifold gasket (connection to block). I realize it could be worse than this. 

My question is--knowing the history you do about the risers, is it more likely the failure is in the riser than in the manifold itself?