In addition to the light mentioned in the previous reply by Charlie99 , there are different types of signaling devices which emit a loud audible tone to make contact over long distances.
The online stores or LDS's carry these devices, which go under the trade name of Dive Alert, among others. :wacko:
Here's an excerpt from an on-line retailer's website:
"This ultra loud air horn attaches to your low-pressure hose in-line with your BC inflator. With the touch of a button, a signaling blast can be heard for over a Mile. A must for night or open seas diving".
At night, a combination of a light and audible signal would seem to enhance your chances of being seen by the boat.
To recap: two chances of being noticed - a light and a noise.
Good luck.