No boosts that I am aware of. I have never encountered a 1 or 2 star serpent (not sure if they exist?) but if it were it's probable that damage output would have been much higher. It's also probable that your boats had some damage you forgot to repair. I always carry some wood with me in my cargo hold to periodically pull over and repair the boat. If you equip the hammer while on the boat, you can catch a glimpse of the health status of the boat so you know when you should pull over or not. Do you use any mods or are you playing vanilla? This could just be really bad luck?