Its fairly academic now as its all been removed, both inner and outer skins were irretrievable, and the transom was already being replaced before I saw this gem, just posted for your amusement really.
The garden hose used to go between the fittings on the inner and outer skins, I think its original. It seems that when the first transom replacement was done (bodged) the hose disintegrated and they just left a void in the ply completely unsealed.
The inner and outer skins have been completely removed since these photos, unfortunately the stringers are rotten too, so will need partial or complete replacement.
I'm roughly following the west system guide to replace it, by gluing the new core into place with 404 filler, then fabric on top.
Not entirely sure what to do about the stringers yet, replace them entirely or just the rotten bit (if possible)? and attach them before or after laminating the core?
further photos below: