I tested your idea:
4mm poly rope with 2 conductors as with the nylon version.
In and out of fresh water.
Grams/m IN OUT
Polly 8.3g 26.7g
Nylon 10.5 22.5g
The poly rope seems to be about 6.5mm diameter and the nylon is about 5mm. I am still leaning towards the nylon to reduce drag. I also prefer the cord -think it might last longer.
I am thinking about making the floats out of barrier pipe sections with blanks in either end maybe with an additional o-ring on the seat.
Or maybe just heat the ends and squash them together.
Polypropylene cord dose exist but the only 3mm cord in 100m length i could find was going to cost £50.