What is Ocean Data Acquisition Systems OR ODAS buoys?

Ocean Data Acquisition Systems (ODAS):

The term Ocean Data Acquisition System (ODAS) describes a wide range of devices for collecting weather and oceanographical data. The systems vary from ocean-going vessels, such as Ocean Weather Ships, to plastic envelopes and drift bottles for measuring currents.

Buoy systems carrying instruments are however the devices of most concern to the mariner, and these may be expected to become more numerous each year.  They are either moored or drifting, and may have instruments either in the float or slung beneath them to any depth.
They are coloured yellow, marked “ODAS” with an identification number, and carry a small plate showing whom to inform if the buoy is recovered.
Moored buoys may be as much as 12 m in diameter, 2–3 m in height and 18 tonnes in weight, and may be anchored in any part of the oceans, irrespective of depth.