The Sea Hag's Horrific Appearance feature lets creature avert their eyes to avoid its effects:
Horrific Appearance. Unless the target is surprised or the revelation of the hag's true form is sudden, the target can avert its eyes and avoid making the initial saving throw. Until the start of its next turn, a creature that averts its eyes has disadvantage on attack rolls against the hag.
The Sea Hag also has a special action called Death Glare:
Death Glare. The hag targets one frightened creature she can see within 30 ft. of her. If the target can see the hag, it must succeed on a DC 11 Wisdom saving throw against this magic or drop to 0 hit points.
It's unclear to me whether averting your eyes from the Sea Hag causes you to not see the creature - there is no mention of that in the Sea Hag's stat block, just a disadvantage on attack rolls against the hag.
This contrasts with other creatures from which you can avert your gaze, such as the Medusa, which explicitly say you don't see the creature while averting your eyes from it:
Petrifying Gaze. Unless surprised, a creature can avert its eyes to avoid the saving throw at the start of its turn. If the creature does so, it can't see the medusa until the start of its next turn, when it can avert its eyes again.
Does averting your eyes away from the Sea Hag causes you to not see it, thus avoiding the effect of its Death Glare?