Birds are biologically afraid of snakes.  Makes perfect sense why.  You don't like birds on your boat.  Makes perfect sense why.  Therefore, you want to put snakes on your boat.  Well, rubber ones at least.  They are an amazing deterrent!  Bigger boats may need more snakes, we recommend one per visible area.
 
Caution: Do not use around humans that are easily scared, startled, have heart conditions, are pregnant, or children who may be easily scarred.  You want to scare birds, not humans or pets.