Well, never considered that that piece/seal might just insulate the water tube a bit from exhaust gases, but who knows. Perhaps this piece keeps hot exhaust gases from blasting down on the water pump through the water drain. That piece is used on the outboards with the open inner exhaust tube that does NOT seal against the bottom of the inner exhaust housing. This piece simply blocks the conventional inner exhaust "wall of water" seal, much like the flared water tube grommets in the water pump.
Do not use this piece on any engine with the sealed inner exhaust tube or water will be trapped in the inner exhaust tube and might back up into the powerhead through the exhaust ports. You will note that I do not call out just the 15hp, because the open exhaust was used on the 9.9 models in later years. And, like everything else, who knows what changes have been made to a particular engine, especially when people are converting 9.9s to 15hp…So, the best bet is to simply look up inside the exhaust housing to see which style inner exhaust tube is present.