These were recently added to the server templates in official map (with the map editor) dated 1/7/2019.
For example, if you look at the POLAR template, you will now find:
"(NPCSpawnEntries=((NPCsToSpawn=(BlueprintGeneratedClass'/Game/Atlas/Creatures/MeanWhale/MeanWhale_Character_BP.MeanWhale_Character_BP_C'),NPCsSpawnOffsets=((X=0.000000,Y=0.000000,Z=-700.000000)),NPCsToSpawnPercentageChance=(1.000000)),(NPCsToSpawn=(BlueprintGeneratedClass'/Game/Atlas/Creatures/MeanWhale/MeanWhale_SeaMonster_Character_BP.MeanWhale_SeaMonster_Character_BP_C'),NPCsSpawnOffsets=((X=0.000000,Y=0.000000,Z=-700.000000)),NPCsToSpawnPercentageChance=(1.000000))),NPCSpawnLimits=,MaxDesiredNumEnemiesMultiplier=4.000000)",
Squid is also there under desert. Each of the epic ocean spawn is assigned to a climate in the official map.
"OceanEpicSpawnEntriesOverrideValues": "(NPCSpawnEntries=((NPCsToSpawn=(BlueprintGeneratedClass'/Game/Atlas/Creatures/Squid/Squid_Character_BP.Squid_Character_BP_C'),NPCsSpawnOffsets=((X=0.000000,Y=0.000000,Z=0.000000)),NPCsToSpawnPercentageChance=(1.000000),bForcePreventSpawnOnOcean=True)),NPCSpawnLimits=,MaxDesiredNumEnemiesMultiplier=4.000000)",
Use the Server (or map) overrides to add your mermaids. The mermaids are only on the maps with PVE islands (quest islands) if you follow the official map design and the override will probably remove the other critters (hence Override...)
Or, combine and mix to your desires.
I posted each of the individual spawner overrides from the official map in the walkthrough: https://www.drac346.net/2019/01/atlas-dss-guide-useful-tips.html