I finally finished my Towfish project. Although there were some startup issues, I am very happy with the result.
The body is made of aluminium and the nose is stainless steel. I mounted the transducer inside the body to avoid the transducer to act as a hook if I get to close to the bottom. The cable is a standard 50m Cat7 cable which I manage to fit inside a PVC hose after a little struggle.. Along the cable I have a 4mm wire with a D-ring for every 5m.
I have only tried it twice, but so far I think the result a OK. The attached screen dump is not the best shot, but the rest of the recordings I have only show sand and stone ;-)
Thanks to all of you for the inspiration and wisdom that lies in this forum.