If your monofin/ tail is smaller that the competitor size ( 30 cm I think?) you can do a Gate check- You carry it with you through security and give it to the flight attendants at the plane door and they put it in last with all the strollers (mark it fragile and ask them not to put anything on top of it!) Call your airline before hand and see if they can put a note in your reservation or just tell the person you're checking in with that you need to Gate Check your fragile sporting equipment.