Two people have been rescue from Sully Island after repeated calls from the RNLI for the public to stay away from beaches.
The charity said it is being forced into the "impossible situation" of choosing between lifeguard and public safety.
The RNLI crew at Barry Dock launched a lifeboat just before 8pm on Wednesday after receiving a report from HM Coastguard that two people had been cut off by the tide.
Sully Island, between Cardiff and Barry in the Vale of Glamorgan, is only accessible when tides are low.
The island is 400 yards from the mainland village of Sully.
Mark Dowie, RNLI chief executive, said: "Despite our warnings that there were no lifeguards on patrol this weekend, crowded beaches, hot weather and big waves meant our lifeboat crews had their busiest weekend so far this year. At least two people lost their lives.
"This puts the RNLI in an impossible situation. With thousands flocking to English beaches now lockdown restrictions have been eased, we must strike a balance that keeps the public and our lifeguards safe.
"Rolling out a lifeguard service – especially in a pandemic – is not as simple as putting a lifeguard on a beach.
"We’re asking everyone to help manage an impossible situation, so please follow our safety advice and think before you head to the coast."
On Friday the First Minister will confirm the changes - if any - to lockdown rules and social distancing in Wales.
Residents of England can now drive to other destinations for day-trips, whereas people in Wales are still being told to stay local.