What is the difference between hydrography and oceanography?

Roughly, whether the water of interest is above mean sea level. Study of the Dead Sea and similar lakes also comes under the rubric "hydrography".

'Hydrography' is sometimes used synonymously with 'hydrology', but hydrography for the seas refers to the gathering of data to aid safe navigation, and as a development from that, wider uses to support exploration and development or environmental management.

Whereas oceanography may be seen as a science for the better understanding of the natural world, hydrography can be regarded as applied science supporting economic development and protection of Natural Resources.

There are areas of overlap: tidal studies, water quality, working from boats and ships. There are areas special to hydrography, such as geodesy (the shape of the Earth), datums (reference surfaces, both horizontal and vertical), applications to an individual country's mapping systems, measurement science (such as the technologies and deployment of survey instruments), boundary demarcation, and others.