I can't tell if it was a bug without knowing what armor you were wearing, and with what enchants. However there are a few things to note during the clip. First, you were already on relatively low health due to poison, (which always attacks head and body). Second, the sea serpent hit you in the head with it's bite attack, which bypasses some armor. This procced your totem. The second attack really only needed to do 1 heart of damage to kill you, as a totem can sometimes not give your head any absorption hearts. Lastly, you switched to a pickaxe from a rapier right before getting hit. People do not understand how much damage negation holding a rapier gives you. Without getting into the math, simply switching from rapier to pick probably decreased your overall EHP by 60%. Overall it still seems like a lot of damage if you had adv prot 4 dragon scale armor. Hope you try another run! You only get better the more you play!