Yes you can tan much faster after being in the water or even near the water.
The water reflects and magnifies the sunlight and directs the sunlight to your skin which causes your skin to become tanned much faster.
Being in Sea Salt water can also help you tan faster so after being in the salt water you can lay out and get a tan before drying off as the salt in the salt water can also allow your skin to tan faster as well.
You could also bottle some sea water that is salty and use that on your skin to help you tan faster when away from the beach.