You don’t necessarily tan faster in water or laying out. This is because water reflects UV rays, so you may not be getting as much exposure as you think if you’re actually in the pool. But when you’re lying on a lilo, your skin will be in direct contact with the UV rays from above and reflected from the pool, meaning you absorb more of them. So, it depends on the circumstances. The best way to tan is gradually, no matter how you do it.