Akin to how Husks in water turn into Zombies, and Zombies in water turn into Drowned,

• Frostbitten should thaw out back to regular Zombies after 30 seconds in a biome with a temperature value higher than 0.1/after being set on fire for more than 10 seconds.
• Swampies should clean off into regular Zombies after 30 seconds underwater.