Animals go where the food and F*&%ing are good. The rig may be hosting a small ecosystem that has helped grow a food source (like an artificial reef can become an oasis in an ocean desert area) or chemicals released from the rig (even poop released in the water) may be attracting the food source.
Or the rig is home to a seal bordello... I'd like to think the food source is a more credible possibility though.