MBC has collected adult and larval fishes from the vicinity of a crude oil spill off Huntington Beach. These studies included sampling of larval populations in coastal, harbor, and estuarine environments, using stratified sampling of the water column with surface, **mid-water**, and bottom nets.

http://www.mbcnet.net/qualifications.php

Typical missions of the 329-foot-long (100 m) T-AGS 60 vessels may include oceanographic sampling and data collection of surface water, **mid-water** and ocean floor parameters; the launch and recovery of small boats known as hydrographic survey launches (HSLs); the launching, recovering and towing of scientific packages (both tethered and autonomous) including the handling, monitoring and servicing of remotely operated vehicles (ROVs); shipboard oceanographic data processing and sample analysis; and precise navigation, trackline maneuvering and station-keeping to support deep-ocean and coastal surveys.

http://en.wikipedia.org/wiki/Naval_Oceanographic_Office

Stemmann, L., G. A. Jackson, and D. Ianson. 2004. A vertical model of particle size distributions and fluxes in the midwater column that includes biological and physical processes. I. Model formulation. Deep-Sea Research I 51: 865-884.

http://oceanz.tamu.edu/Directory/Faculty/Phys/jackson.html

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Note added at 53 mins (2011-02-08 22:48:26 GMT)
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Background
Hover capable Autonomous Underwater Vehicles such as Seabed and ABE are uniquely suited for conducting in-situ chemical oceanographic measurements.

Specifically they combine

* An ability to conduct very precise (navigation accuracies close to 1m); repeatable surveys over large areas (10km2) near the air sea interface, in the mid-water column and close to the seafloor.

http://www.whoi.edu/page.do?pid=7795&cid=16132&c=2

equal to 'mid-depth' there are many hits for medium depth in the context of water sampling.....just another option.