Sailor's "Stop!" (5)
(Other definitions for avast that I've seen before include "Stop! (naut.)" , "Stop, nautically speaking" , ""stop" signal" , "order for tar" , "Stop at sea" .)
(Other definitions for seaport that I've seen before include "Dover, say" , "Sydney perhaps" , "Place with harbour for ocean-going ships" , "Coastal town with harbour" , "access to drink" .)