I'm in a ProStar 190 boat share too, and that boat has blanking plates at the transmission end of the manifolds. I'll get some photos up today. I think maybe the previous owner has got a bit carried away!
Bouyhead as mines a salt water series the intake manifold and cylinder heads are cooled with antifreeze (indirect), so that coolant doesn't exit via the exhaust manifolds. I think it's just the raw water that goes into the manifolds. I'll get some pics uploaded.
'92 190 Prostar 260hp Indmar.