Beach Adventure is a brush adventure that can be used to brush warp to shore from the sunken ship. This is useful for a route that uses sunken ship to set the in-game time to night in order to spin the whirlpool galaxy. It can be challenging to arrive at Watcher's Cape before night ends. Beach Adventure can be used to sacrifice some real time in order to preserve the night cycle. Night does not pass during the brush menu, therefore this brush warp can allow you to travel "instantly" without night passing. This strat was developed by popprocks, who explained and demonstrates it here: https://www.youtube.com/watch?v=gtsRn052RzQ
Beach Adventure is not mandatory to reach Watcher's Cape before night ends. However, any mistakes made by the runner will be very likely to miss spinning the whirlpool galaxy in time, which would have catastrophic outcomes. Even when doing Beach Adventure, the runner still has a very narrow window for error.