One of the more popular temples is Pura Tanah Lot. This temple is perched on a rock. When high tide, the temple looks as if it is "floating" in the middle of the sea.
When I came here, it was still low tide, thus we could walk until the bottom of the rock. Before reaching the rock though, there's a cave where you can see a snake which is believed to be holy. Once reaching the bottom of the rock, there is another cave with streaming water also said to be holy. You can receive blessings here for a small donation.
Besides its unique location, this temple is also flocked by tourists because it offers a stunning view during sunset.
During High tide (Pic source: http://m.wikitravel.org/en/Bali)