Butyl Tape for Below Waterline
I am not sure I would have gone that approach.
I have no issue using Butyl Rubber above water line for deck hardware, but for I would never use it below waterline for thruhulls. I would have used Sikaflex 291 or some other Brand Name of that type of material. Butyl rubber has NO tensile strength and weak adhesion, so all of the load on your stern tube is now on the bolts. Also, since the rubber has weak adhesion, if any there is any really poor bonded areas, water can push past it. Unlike deck hardware which has no water pressure, the stern tube has the pressure of the water holding up the boat. With an adhesive, like 3M 5200/4200 or Sikaflex 291, the the adhesive you have much more adhesive properties and the adhesive distributes the load across the bonding surface.
If I was you, and the boat isn't launched, I'd rebed the stern tube with Sikaflex 291 or 4200. If it is in the water, keep and eye an one it and replace it at the end of the season.
If you liked my advice/response, give me some rep points. If you didn't......well what do you expect, it was free.