Just as I was getting ready to upload this this, I thought that surely someone else has found that out waaaay before me, so i googled a bit and naturally found a PDF already from 2006 (by Patrick Rowe from the Woods Hole Oceanographic Institute) which explains everything much better and shows how to get real waterthight cable joints/splices with some ordinary hot glue and shrink tube:
I've been following Hackaday since at least 2010 and do not recall this 'hack' being mentioned somewhere, a quick search of the Hackaday Blog didn't turn up anything either, so I thought it would perhaps be a nice idea to bring attention to this technique.
(or maybe it's so common that everybody but me already knew about it?)
There isn't much I could contribute further, the above mentioned PDF does a much better job. Thats all...