Holidays and working have eaten nearly all the time I have had to work on the mystery as to why the ark sank. A bit of a shove on the pool didn't reveal any obvious leaks I'm sad to say, so it's back to guesswork.
One possible issue is the hull is in two parts. The top ring is sperate from the bottom, so it the boat reaches that depth, it's going to fill up very rapidly with water.
Ideally, I'd have separated the components and glued them together. With the motor fitted, this isn't an option so it was time for a less appealing Plan B. If I can't stop the water getting into the hollow bits of the hull, at least I can stop it getting into the main bit.
Along the front edge (right-hand arrow above) I ran copious amounts of Poundland epoxy glue. It splodges nicely into place and will stop any water.
Under the top edge, the hull is hollow, so I filled it with Milliput. Then, because I've never got on with the stuff, coated it with more epoxy.
I don't know how much all this will help, but I reason at least if the boat starts to sink, it will do so slowly.
Now I just need to find time to get back out on the pool and really sail this boat in a safe environment to see what else happens. There are some electronics to replace first though as the servo is chattering, which might be the servo or the receiver. Which I wish I'd waterproofed in a balloon.