Some pools might be heated and some might not, depending upon the ship. Many of the newer ships also have a retractable dome over one of the pools, which undoubtedly is closed and heated in cooler climes.
Just got off the Sea Princess! The swimming pools (3 total - I think) are 90f and the (somewhere around 6) hot tubs are 100f. They are open 24 hours a day, unless the sea is choppy. It was pretty choppy one day last week and they had to close the two main pools on the Riviera deck because waves were splashing about 20 ft into the air! I like the pool in back on the Lido deck with the dome over it. It's more protected from the wind.
The aft pool has a canopy dome and is somewhat protected but is still open to the chilly Alaska air. It was our favorite spot on the Dawn Princess when we transited the Panama Canal. For Alaska, however, I think that pools like those on Celebrity's ships or in the Solaria on RCI's fleet of ship are more likely to get regular use regardless of the outside temperature