[Mr. X]

I have tried some rope similar to the above (readily available in the UK from hardware outlets, usually light blue in colour and often used by truckers) but I found it rather floppy & tended to snag/knot more that I was comfortable with.

Rob Allen sell an inexpensive polyester rope (around7 or 8mm I think) in olive green or hi-viz orange, which is quite affordable (I think it was maybe £7/$14 for 20m with a spliced look at each end) and compact. Various lengths are available & it's easy to splice a new loop if you decide to cut one down. Olive green would be my suggestion. It's quite stiff too, which is good for this use I think. This is what I started with and have gone back to.

There is also a very cheap bright yellow braid that is readily available. It's a pity shiney/plasticky & stiff to use. It'd probably do the job but bright yellow is rather conspicuous.

Some washing lines float, some don't -- not a bad place to start.

The sealed aquarium air pipe seems quite popular (clear, stiff, floats, affordable if you shop around).