Freediving Rope

Yes, climbing ropes are rather expensive, and additionally they are heavy and when soaked with water they take long time to dry, hence being even heavier for the transport, cath easily mold, and sink in water, which makes them then more difficult to manipulate if you are not on a boat. You also better take a static rope (stable length also under load). In a marine shop you may perhaps find something better, but those shops are usually rather expensive too, and the rope may be an overkill for your needs. You will probably find a suiting rope in a hobby store near you. Apneaman sells excelent ropes, that are light, thick enough (10mm and 12mm diameters), do not soak with water, dry fast, floats, have a stable length, and are well visible. You can put up to 100m of it into their floats and do not need two persons for transporting it: rope Lannex PPV 10mm - APNEAMAN s.r.o.

My favorite is 12-14 mm double braid polyester rope.
If you have to travel with it perhaps go thinner but then its not so nice in hand for free immersion and hangs.
I like the thicker rope because its easier to see with no mask dives.
Always wash and hang the rope and dry it thoroughly after use in salt water.
Keep it protected in a bag or tub to extend the life.
If you mark it properly its a lot easier to set the depth too.
Depending on the depth your diving sometimes its cheaper to buy the whole reel.
Check ebay. I got my 100m for 140 aus.