Mid and West Wales fire service and the RNLI have issued advice as they predict a busy bank holiday weekend.
It includes supervising children, not using inflatables in strong seas and swimming only at beaches patrolled by lifeguards.
RNLI lifeguard manager Phil Davies urged people to heed their advice.
"We want people to enjoy our beautiful coastline, but make sure they do it safely," Mr Davies said.
"Conditions can change quickly at the beach, so it is really important to respect the water and take extra care when visiting the coast."
Steve Davies, of Mid and West Wales Fire and Rescue Service, added: "We cannot stress enough just how dangerous the sea can be, and how your life could change in a second.
"We want you to enjoy the wonderful beaches that the Welsh coastline has to offer safely and incident free this weekend."
Safety advice includes:
Lifeguards will patrol beaches at Camber Sands, in Sussex this weekend after five friends died during a day trip to the coast.
Their deaths on Wednesday come after six others died around the UK coast last weekend.
Beachgoers have been urged to stay safe on the coast this weekend following 12 sea deaths around the UK in the past week.