What just one shot me through MoM? Left with mana after death.

I'm in act 3 (level 40) in The Drowned City, I just died in one shot but was left with nearly full mana.

I've got MoM, 800 hp, 400es, 650 mana.

I was stood in what looked like a pool of water.

I'm trying to replicate it, but the water is doing very little damage.

Your character drowned in the water by the skill of the ancient spirit of drowned city (the moving water bubble, not the water swirl).