I doubt that putting yellow markers out could construe 'an enticement' to swim. If there was the slightest risk that councils/EH were actually responsible, they'd have someone checking your swimming certificate to see how well people can swim before they go into the water, or provide lifeguards (i.e. like a swimming pool - which probably does form an 'enticement' to swim!!).
Having said that - we get muppets swimming in the main harbour channel, even though there are boats and jetskis coming and going at speed, and there are red flags flying permanently on that section.