The charity said it had to temporarily withdraw the lifeguard service from coastal beaches on Friday.
It said it had made the "difficult decision" due to staff shortages.
Ingoldmells and Sutton on Sea remain will remain without cover for rest of season but at Skegness and Mablethorpe beaches they have now been reintroduced.
Arun Gray, lifeguard supervisor, said: "Skegness and Mablethorpe are back open now as normal.
"They were only closed on Friday but we managed to get a lifeguard service up and running for the bank holiday weekend. S
"Sutton and Ingoldmells are closed for the rest of the season."
Daniel Cooper, of the RNLI, warned only "very competent" swimmers should enter the water at unguarded beaches.
In an emergency dial 999 and ask for the Coastguard.