Aluminum is the wrong material to use for risers, which is all there is to say on the subject, except that some Yanmar engines have them.
Good luck is needed here.
The risers bolt to the exhaust manifold, and because the two parts expand and contract at different rates, it's hard to maintain a constant gasket seal. If water is leaking to the outside, it is also leaking to the inside. Count on it. This may not wreck your engine, but salt water under the riser gasket will corrode the gasket surface on the exhaust manifold. A gasket cannot seal to an eroded surface.