This is so cool D!!!!!

No rocket science Alessandro😉
Just a hangover from several years working as a Service Engineer and later Chief Engineer of a Test&Development Department.
Ciao, Doug😎

Glad to help Doogle👍
Look forward to further progress reports.
BTW: Doug and RNinMunich are Me and I😁
BTW 2: I note that you have written your Calypso thread under the 'Website Related' category.
That is actually intended for queries and info about the Website operation.
There is also a category labelled 'RC & Electrics for help on queries such as this one.
Cheers, Doug😎

Ah, see the problem 😕.
Thanks for the advice, will let you know when I venture outside again.
Going to have a cup of tea 🍵 as been mending stuff for the neighbours. Thanks Doug and RNinMunich for your help 👍

I wonder if I only saw the text because you added the photos later, or did I only see the text because I'm going stupid?
I think the second one, ahahahahahah.

Anyway, I repeat, a big round of applause to Doug and his spirit of observation and research.

Amazing Doug, your eye never fails.
Well done I admire you.

Hi Doogle.
Re: " Only problem is that both motors work individually off the receiver but when linked via a Y lead, then nothing."
I see your problem😉
The Y cable output to the ESC on the left of your picture is wrongly connected.
The input from the Y cable to the ESC is reversed. Thus short circuiting the RX pulse signal to ground. This disables both ESCs via the Y cable. See attached extract from your photo.
It should not be possible to connect it like this as the plugs and sockets are keyed.
You must have had to use some force to push in the Y cable plug the wrong way round.
Turn that plug around and try again. With luck you may have got away with it and all will work.

BTW: When connecting two ESCs to one receiver you MUST cut one of the red wires between ESC and RX. Otherwise the BEC voltages from both ESCs are connected together, which causes a conflict which can damage the ESCs and/or the receiver.
Cheers, Doug😎
(BEC = Battery Eliminator Circuit in the ESC which provides the DC power to the receiver. ONLY ONE such connection is needed.)

Great Doogle, great recovery.
It went better than I thought, good thing.
Change that cable now.

Popped down to the lake this morning, one of the guys has very kindly 3d printed the submersible and other pieces which were lost when she sank last Sunday. All the electrics have been dried out and work. All tested individually and then bench tested in the warmth of the lounge. Only problem is that both motors work individually off the receiver but when linked via a Y lead, then nothing. Guessing the lead plus a spare one is faulty. Not my strong point electrics, one of the guy's has kindly offered to come round and show me how to set up my flysky so that I can mix both motors but until then I'm happy with a Y lead.
As you can see, the 3d print is excellent and I've been very fortunate to not suffer more damage and loss other than a few bent railings and fixtures which I can now repair.