mtg 60 card deck breakdown Mighty Mill-60 Deck-rares-magic the Gathering-mtg-blue-black-fast-ready to Play Everything You Ever Needed to

2 Ride The Shoopuf All Aboards

That said, replacing either the basic Island or an Underground Sea with whichever blue land allows you to splash into a third color can be perfectly fine for sideboard answers and is actually what I do in Legacy for Bant, and what I sometimes try in Modern for Bant as well

In fact, while the two cards certainly have some synergies, you could very easily view them as being entirely separate, where it not for the fact that they are simply two sides of the same coi card

If its your first time competing in a tournament, I would definitely double-check the legality of all your cards, but otherwise, take it easy