Everyone says, "Don't swallow." Easier said, then done. Having raced for over 20+ years, the one stroke that I swallow the most water is the butterfly. You can do some tips to reduce and/or cope with that dreaded mouthful of water.
Tips:
When you breathe, place your tongue on the top of your mouth. Your tongue gives you a barrier to hopefully stop the water from just gushing down your throat so you can spit it out.
During butterfly, if you think you'll be at high risk for swallowing water, breathe to the side. Typically I would breathe to the side off the start, and off the turns, then I would return to breathing normally.
Don't exhale all your air. I try to exhale only 50% of my air in my lungs, so if I can't stop the water from sliding down my throat, I can maintain my speed without that breath. (pro tip: my next breath would be to the side to guarantee fresh air)
Use the air you have left to either cough or blow air out your nose to dislodge any water that might have gone down your airway.
Pro tip:
Don't be scared to be in a situation to practice this. My coach used to have me swim butterfly with people splashing me. The key is not to panic.
Good luck out there, and keep swimming fast!