No, I don't think it is an amphibious mouse
Tim, I have only had it for a month or so and I am fairly certain there was no sign of this damage when I bought it, it certainly didn't appear to be leaking till about ten days ago and I haven't been in any water shallow enough to hit a rock since I have had it.
Chris, fairly sure it is not a mooring line as I have had a mooring line running over the tube and where it was rubbing on the tube at one stage it restored the full "orangeness" without doing any damage, there is no sign of this where the damage is.
I am pretty sure it is a fatigue failure (even though I don't understand why) which worries me because even if I can patch it successfully, it might fail just next to the patch. You can't see it on the photos but there is a slightly "weakened" appearance following a line around the radius of the tube, going through all three failure points.
I have found a couple of people who think they have got some glue which they are going to dig out tomorrow (no idea what type it will be though).
This Hypalon stuff must be a really odd chemical makeup, I had a thought earlier and went and got some glue we had at work, used for tubeless tyre patches, this stuff is to ordinary vulcanising cement what Superglue is to Pritt-Stick, and normally sticks to anything vaguely rubbery like SH&& to a blanket! Wouldn't stick two off-cuts of the repair material together though... peeled apart very easily.
I think what I am going to try is a small patch over the immediate leak to try and make it air tight so I can pump the tube up, and once there is some pressure in it (so I have something to press against) then try a bigger patch over the whole area - I have a chunk of fabric about 1 foot x 3 feet for repairs.