Drunken fools are unstable. This could have multiple meanings, but in this verse in Proverbs it actually only means that they are physically unstable. The drunker the fool gets, the more he is unable to control himself. He will eventually stumble around and fall down a lot. This is because of how alcohol affects both his brain and his muscles. Both are adversely affected and are hindered from functioning normally. In dealing with the previous verse we saw how both sight and muscles are weakened and disfunctional when the blood alcohol level of a person rises. The lack of sight - and the inability of the brain to handle the direction and regulation of our muscles is what causes this instability.
The actions of the drunken fool are compared to those who lie down in the middle of the sea. This first phrase refers to one who has sunken down into the sea. Similar words were used of Jonah and his fall into the depths of the waters in Jonah 2:4 - thus this is a picture of a person who is either drowning or in danger of drowning. He is lying in the midst of the sea - which is literally the heart of the sea. In his intellectual confusion he slowly passes into a state of unconsciousness - where he is no longer in control of his life. The drunken fool drinks until he is fighting a battle with being conscious. Even if he is conscious physically, often the drunk cannot remember what he did the night before. Some fools who have lived like this even find that they have injured someone, impregnated or have been impregnated by someone they don't even know, or in severe cases have even killed someone. Truly just as the man sinking in the heart of the sea is no longer in control of himself - so the drunken fool has lost control and places himself in grave danger.
The second statement here is that he is like someone who lies down on the top of a mast. This place mentioned here is the crow's nest or lookout mast on a ship. These are not made for anything except standing. When a man is standing in one he can hold on and can determine direction and warning for a ship. For a man to be lying down in such a place is foolish on several counts. First of all, doing this means that he is most likely lying down on top of the nest itself - with no way to hold himself stable. Since the motion of the ship on the sea is greatly exaggerated here, he is more likely to be thrown from the nest into the sea or to his great injury or death on the ship below. The drunken fool is similarly in danger because drunkenness greatly impairs his ability to protect himself and guide himself safely wherever he is. He too is in danger of injury or death in his drunken state. The second way this is foolish has to do with being watchful. If you are lying down on top of the mast you are not watching. The ship could run aground and face terrible problems when its lookout is asleep or lying down on the job. If enemies approach he is supposed to be the first line of warning. But since he is lying down on the job - he is subjecting the ship to a condition of having no lookout at all. The drunken fool is similarly abandoning watch over his life, his family, his friends, and most especially his testimony. He is a man adrift with no one at the helm - wandering through life with no direction as well as no protection. Some who choose to live this way, ruin their lives in the course of one night.
The wise man does not impair himself willingly. He keeps his wits about him and keeps watch over his life and testimony. He knows that drunkenness will severely blind him to everything around him and also will leave him without any discernment or prudence in his decision making ability. Therefore he avoids drunkenness like one would avoid the plague. He knows that rather than being drunk he is to be filled with the Holy Spirit. That way he has a teacher and a guide who can keep him safe. He knows that under the influence of This Spirit he will not have to wake up in the morning wondering what happened and whether he will need to be ashamed of the previous night's activities.