The California Academy of Sciences has a live Shark Cam on YouTube. It shows an HD underwater view of a shark lagoon, and it even works at night (in black and white night-vision mode).

The display is very soothing, though the actual shark content is quite low. If you like watching rays cruise by—and who doesn't?—you'll love this. You may also enjoy putting it on as soothing background video. Chill out and watch:

If you're curious what species you're seeing, check this field guide. Long story short, it's tons of rays (of various species) and diamond fish. If you want more sharks, check out the Monterey Bay Aquarium's Shark Cam.