Instructions
Visit Shoal Cave during either tide.
Save your game while still inside the cave and turn it off.
Load your game after waiting for the tide to change. The low tide is from 3:00 p.m. to 9:00 p.m. and 3:00 a.m. to 9:00 a.m., while the high tide is from 9:00 p.m. to 3:00 a.m. and 9:00 a.m. to 3:00 p.m. When in doubt, add six hours to the last time you played to make sure the tide has changed.
Load your game. The cave will still be at whichever tide you saved on, even though the tide should have changed over. This allows you to explore the cave and get everything you can during the particular tide.
Exit the cave then immediately walk back in. The tide will now be the opposite tide from your last save. You can now continue to explore Shoal Cave and reap the benefits of the current tide.