I've met this thing 3 times, first time, I walk to the NPC shops and I see this guy dead, and I'm like, thats not good. I zoom out my camera and I find a SEA HORROR at the top of the stairs, so I just back away fast, but it followed me to the cabin killing everyone.
Second time I'm fishing when the Sea Horror appears, I smarten up and I run to the Captains Cabin thing (I hope I was safe there) but the Sea Horror actually opens the door and kills me. Luckly a whm right beside me raised me, I didn't know why the horror didn't attack the WHM till she told me it detects on sound and she cast sneak on herself.
Third time I smarten up really good and I just keep casting sneak on myself(there were tons of other mages on the ship that could cast sneak and everyone was sneaked), and half way the horror just disapeared!.