Understanding Cord Disintegration
Dolphin cords are made of a rubber clad multiconductor. The blue rubber cladding, over time, seems to degrade and gets tacky. This is completely normal for a rubber material submerged in chlorinated water and exposed to heat and UV. Look at these pictures -
Notice the difference in the cord - there is a part that is almost exclusively on the deck all the time (the dirty looking part of the cord that is otherwise uncompromised) and the part that is in the water all the time which is rough, white, tacky and chalky (you can rub the rubber off the cord with your hands). But it's not just about what is submerged in water or not; look at these photos -
Notice how the part of the cord that is under water BUT NOT exposed to sunlight, still looks and feels brand new. You can see it dramatically around the rubber grommet where the cord enters the robot. So it is clear that it takes several factors - water, chlorine, and UV - to cause the cord to degrade. And this is to be expected as rubber materials easily undergo hydrolysis and breakdown when exposed to a strong oxidizer and water.
A practical take away from this is the following - removing the robot from the pool when not in use will likely improve the longevity of the cord. I do typically take my robot out and, unlike a lot of robot owners, I have almost no issues with cord tangle or cord splitting open.