Hi Daniel, and thanks! It is natural, though it isn't so much a cave as just two rather large rocks leaning on each other. You're probably right about the cropping, too. Composition was difficult, as it involved me running back and forth from the camera position whenever a wave would roll in and cover me up to my ankles in water, so I could usually only stay for about five seconds each time! :)
Now there's an interesting way to do a sunset. Wonderfully composed but I wonder if cropping a little more from the right would help? I'm interested to know if this was a natural cave or an added one? If it is the real thing then, well done for such good positioning, with the sun it's just about perfect. If it's an addition, well done for having the vision to add this effect, it really works. Either way, great shot, congrats.