We haven't sailed on RCL however have sailed on NCL, HAL, Celebrity and the pools were heated. HAL's Zaandam had fresh water and was covered as well, Celebrity Mercury had a smaller heated covered pool and one of the 2 outside pools was heated and salt water. NCL Sky (no longer up there) was outside and heated and salt water.
I'm sure that the Radience will be heated.
So it should be ok to bring the swimwear.