This is a Wandering Sea Anemone, during the day it looks like a ball of baked beans but at night it releases its tentacles to feed. This was found whilst diving in a local harbour, its quite amazing what you can find lurking in structures made by humans. On the same dive I found a Weedy Sea Dragon which is quite rare as they are usually living in depths of 15-20 meters but I found one in about 5 meters.
Canon 5D Mark III, Canon 100mm Macro, Aquatica water housing
1/200, F22, ISO 100