Drag to turn the globe, scroll to zoom. Hover the shore: the dot snaps to the coast and the arrow shows the direction straight out to sea. Click to follow that great circle until it meets land.
Two fingers turn the globe and pinch to zoom. Drag one finger to put the dot on a shore (it sits just above your fingertip), then tap Look across. For more room, use the full-screen button or add this page to your home screen.