My HP assembly came with both cap and barbed hose. I have the cap installed and I measured (5/16) the unused barbed fitting. Barbed fitting part# 22-814382.
It is smaller than 1/8" fitting.

I will yield to you sir

Just looked at the pics ---- 1st. is the hose from the fitting is cracked causing the water leak.. and 2nd is - without hose connected at both ends, its going to pee and or leak water. - 3 rd is --- take the fitting out and a pipe plug goes into the coupler. The size of the pipe plug is .0625-27 npt.

Mercruiser sells it as well -