Metal railings have been stolen from barriers surrounding North Dock in Llanelli.
The removal of the stainless steel poles has left big gaps overlooking the water.
Smaller metal barriers have now been placed around the perimeter of the dock to protect the public.
The area is very popular with dog walkers and those visiting the nearby beach.
Glanymor ward councillor Sean Rees called it "completely unacceptable" as he urged members of the public to report suspicious activity to the police.
"This reckless action is completely unacceptable," Mr Rees said.
"A year ago, railings were removed and thrown down to the river bank.
"However, since then more have now been taken, around 10 in total.
"If anyone sees any suspicious activity, I’d encourage them to please report this to our neighbourhood policing team."
What is vandalism?:
Carmarthenshire Council's head of leisure, Ian Jones, called it "very disturbing" that someone would "put people's safety at risk" by removing the poles.
He said: "This is a blatant act of vandalism and has been reported to the police.
"The safety barriers are there for a reason and it is very disturbing that someone would choose to put people's safety at risk by removing them."
