The irony is: best boatbuilding practice is to do everything possible to make sure that the hull is not biodegradable!
David D asks: "But what to do with older worn out GRP hulls? How can these hulls be either disposed of safely, or repurposed in some way..."
A well-researched and very well-written article by Derek Pilgrim may be found in JRA Magazine #85 (February 2021), pp 43-48, on this very subject. For anyone interested in this subject, it is worth to download.
Here's a better idea - here's how they re-purpose fishing boat hulls in the village of Equihen Plage located on the coast of Northern France.
You can read about it here. I just love that.
Its a pity that it is the superstructure of a boat which usually seems to go first - otherwise an even better idea would be to utilise the hull right way up. I've dreamed for years about getting an old ferrocement cruising hull and burying in on a plot of land somewhere, down to the waterline. Only problem is, I don't have a plot of land.
Sorry Jim - a bit off the topic of Teleport !