ROV

[guido99]

Tactical optical fibre is probably best for your applicaton and is easily reeled and unreeled, this particular brand is 5.8mm O.D. for the 2 core. Unfortunately durability = larger O.D.


http://www.drakausa.com/documents/productPDF/DCB-TACTFOC03s.pdf

[Moonwalker]

Yes already checked the tactical fibre but the diameter is too large. I would go for coax which is just a little ticker and a fraction of the price.
As said before the target diameter is 3mm. Then I would wrap a spectra briad around/next to it for pulling strength. SO the cable only has to be durable for rolling and unrolling and not for pulling strength.
I think I need Loose tube cable for underwater use but what I found so far are all over 6mm thick.