Digital holography has been growing in importance for application to environmental studies in the oceans and lakes of the world. With an imaging resolution using "classical" photoholography of a few micro-meters and recording volumes up to a cubic meter, several "holocameras" were developed and deployed for underwater imaging of plankton and other marine particles. For in-water deployment, however, the weight and size of these instruments restricted their use on advanced observation platforms such as remotely operated vehicles, and limited operational depth to a few hundred meters. Advances made in digital recording on electronic sensors, coupled with numerical reconstruction, led to the development of smaller, rugged holocameras. This freed holography from many of its constraints and allowed rapid capture and storage of images and holographic video recording of moving objects. Although holography is not the only optical method applicable underwater, its ability to record full-field, high-resolution, distortion free images in situ from which particle dimensions, distribution and dynamics can be extracted is hard to match. The current state-of-the-art in underwater holography is discussed, with an outline of some submersible holocameras. We describe one such system, eHoloCam, in more depth and present results from its deployment in the North Sea.