Well, the issue is the $2.00 upper water tube grommet under the powerhead at the top of the exhaust tube. And, no, they had not resolved this issue on the 1984 models. But, this issue usually only affects salt water engines, this one looks to be a fresh water unit. If the grommet was an issue, the engine would be more affected at high speed than low speed anyway. Normally, the overboard indicator spray is kinda weak at low speeds on this engine. It sounds like you are not going to keep the engine, so I don’t think you are going to want to rip that powerhead off, quite a chore on this engine.
Run the engine on a boat, evaluate the cooling system. If in doubt, I think I would just go after that water pump again, perhaps install the complete kit with all the housings and impeller plate. Does the powerhead show any signs of overheating? I think this vintage of the 9.9/15hp was when these engines were on top of their game.
You may want to evaluate the upper mounts though, you wouldn’t want to sell the engine with bad mounts. Replacing the upper mounts is expensive and requires powerhead removal. Needless to say, you could have a look at the upper grommet if you had to pull the powerhead and do the mounts.
Let me know if you have any more questions/concerns, these were great engines: no 100:1/cheap plastic carb/expensive throttle cable. The 100:1 mix was introduced on the 1985 models, I am assuming this is a 1984 model.
PS-What is the strange looking thing attached to the gearcase right in front of the prop?