Re: 90 HP Johnson leaking water at seam
Actually, the water is leaking from your lower mount cover. It is not supposed to do that. If you look at the seam between the midsection and the gearcase, you will see voids where they meet. These are intentional to allow water to leak out between the midsection and gearcase.
I would remove the lower mount cover and try to trace where the motor is leaking. There could be a crack in the midsection.
As for the seam, it is a metal to metal ungasketed fit and leakage there is normal. Actually, you are supposed to use some sealer such as Scotchgrip 847 in the joint when ressembling after a lower unit removal, just for cosmetic reasons. But if it hasn't been done, don't worry because anything coming out there was destined to go out with the underwater exhaust anyway.
As for the mount area. There are large drain holes there that get rid of water that escapes the water pump around the drive shaft hole. It is perfectly normal and expected. In fact if it weren't drained off, it would go up to powerhead level and possibly cause damage.
So, relax your motor isn't broken.