This is a slang word for a female in the Navy made popular by males in the Navy in San Diego. Usually a sea monster is one of the few girls on a ship full of guys. She usually isn't very hot, but thinks shes really hot because she gets hit on 24/7. She might be a slut. You might find her hot and then she becomes uglier as you get to know her better. She usually isn't very nice, usually bitter because of the way some guy did her previously. She might be a dyke, sometimes females in the Navy end up hating males and they end up rug munchers.
Conversation had on 6 month Deployment Sea Tour:
Young Naive Shipmate: "Woah! That girl is fine!"
Older Wiser Shipmate: "Easy young buck! That is what we call a Sea Monster...Your best bet is to stay clear from her! Don't be cast under her Sea Monster spell!"
Young Naive Shipmate: "Dammit! I can't wait til we get back to San Diego!"