You're not talking about dry exhaust, right? Rather, you're talking about the water from the block going to the manifolds and risers, and then getting dumped into the exhaust. If so...
Short answer is that I still don't know. What you're describing makes sense about how it should only go up because it's effectively additive. But one other wild card is that evaporation absorbs heat. Once water, even hot water, is introduced into the exhaust gas, I'd expect there was some rapid evaporation, which would provide a temp driver in the other direction.
Not sure how they'd stack up against each other, though I'm thinking the water leaving the boat would be warmer than leaving the block, just mitigated by some evaporative cooling.
"I don't have time to get into it, but he went through a lot." -Pulp Fiction