Average Atlantians of Atlantis are Size 3 Amphibians (three-to-a-square). Whether they're silly frogmen or eldritch abominations is not too clear, but it is clear that they're strong and clumsy; 12 HP, 3 Natural Protection, 11 Strength, 9 Defence Skill (naked), 8 Precision, and 10's in Combat Speed & Map Movement (also while naked). Originating from the gorges, they have 50% Darkvision, which is handy in deep seas.
Shamblers are middle-aged Atlantians, having grown to Size 4 (two-to-a-square). On top of the innate Atlantian traits (and their low Defence and Precision), they have 22 HP, 7 Natural Protection, 15 Strength, 11's in Morale and Combat Speed while buck-naked, and 14 Map Movement while nude.

The Deep Atlantians (or Deep Ones) are bestial folks with hunchbacks and angler lures. Also Size 3 Amphibians, these guys have 100% Darkvision due to their environment, along with 5 Fire Resistance and 5 Cold Resistance (like a caveman). Many Deep Ones use their monstrous jaws in battle, giving them a bonus attack (if a basic one). Statistically, the average Deep One has 14 HP, 4 Natural Protection, 12 Strength, 12's in Morale and Map Movement while naked, 9 Combat Speed (also naked), and 7's in Defence Skill, Precision, & Magic Resistance.
Shamblers of the Deep are slightly less beastly than their younger Deep siblings, but they still have their bonus attack and their other traits beneath the stat block. The average Size 4 one has 26 HP, 9 Natural Protection, 16 Strength, 7 Defence Skill (still), 8's in Precision & Magic Resistance, an impressive 13 Morale in the buff, and the same mobility as an unarmored human (14 Map Movement and 12 Combat Speed). And they get bigger than this…