Instructions
Travel east along route 124 in the Hoen region. Go north on route 125 just before you reach Mossdeep City. This leads you straight to the Shoal Cave entrance.
Enter the cave anytime during 3 p.m. to 9 p.m. and you will find that it is low tide. This exposes more of the cave interior and allows you deeper access. The tide changes every six hours, so you will have to wait if you reach it during high tide.
Walk forward from the entrance and then turn left before going up again. Take the stairs to your left and exit the cave through the doorway in the middle.
Make your way to the middle of the cave where you will find a ladder. Go down the ladder to reach the next floor. Walk to the right where you will find a boulder blocking your way.
Stand in front of the boulder and use "Strength" to clear it out of the way. Go down the ladder at the end of this passage and you'll find yourself on the icy floor of Shoal Cave.