What material is the hull made off?

If it's somewhat heat resistant, you could save yourself the trouble of removing the propshafts (stuffing tubes).

Run the propshaft on land (one at the time), while running, put in new grease with a syringe, so there's some pressure on the "old "grease.
In my experience, the drive train will get warm within half a minute due to the friction of the propshaft on/in the grease.
If it's hot enough and you keep the pressure on with the syringe, part of the old grease will spill out the bearings (put some paper tissue under it).
Let it cool down and the stuffingbox should be watertight.

Regards, Jan.