Think I found the problem!
For my very first version from ROV I have been using the A2212-outrunners. That means I have to solder the BR-cable onto the wires coming from the A2212-motor. To keep the water away from the connections/soldering I have used two sizes of heat tubing: a small size to protect the soldering and a larger one to cover part of the three wires and part of the BR-cable. I have used shrink tubing with glue.
I figured out the problem from adding a little pressure to the WTE while at the same time putting the leaking cable/motor into a bucket of water. What happened? Well, a few air bubbles quickly showed up in the bucket...
So, all I have to do is figure out a way to "seal" the end of the BR-cable to stop the water from entering... Potting in some way? I don´t know...
Have attached a few pictures below.
1st picture: cables before cut open
2nd picture: took away the large shrink tubing
3rd picture: must be here the air exits/the water enters