Many wall dive sites are in close proximity to more gently sloping reefs and unconsolidated sediment bottoms.
In plan it can be anything from nearly straight to highly convoluted, with gullies, curves, sudden changes of direction, transverse canyons, and offshore stacks.
Biodiversity tends to be higher where there are a wider range of habitats concentrated in a region, and this is generally true for walls, though they may lack organisms that prefer a more horizontal substrate, and those easily dislodged which may fall to relatively inhospitable depths.
[2] Local biodiversity may be higher than average for the region due to wide depth range and variety of habitats.
[1] Currents may follow the face of the wall both horizontally or vertically, in upwellings and downwellings, and there can be strong vortices and turbulence at sharp changes in direction, and overfalls at the top.
Local variations in wall topography may provide shelter from currents or areas of increased turbulence which can sometimes be visually recognised by movements of benthic organisms, like sea fans or seaweeds, with the flow, or occasionally by the predominance of organisms that are suited to strong flow or turbulence[4] A further hazard for divers towing a surface marker buoy is getting the buoy snagged on a shallow obstacle while the diver is in a strong current, as the combination of flow and restraint at the end of the line can cause a diver to swing upward rapidly, which can violate recommended ascent rate and present a risk of decompression illness.
Vertical areas and overhangs may make towed surface markers difficult or impossible to manage, depending on current strength and direction, wind, and wave motion, and a dive leader towing a surface marker buoy may have to stay far enough from the wall to avoid getting the line snagged on a shallower part of the wall.
[5] The other common form of diver contact with the wall surface is holding on to control motion due to sudden or unexpected changes in water flow.
Benthic organisms vary considerably in sensitivity to this kind of contact, but unlike fin strike, the diver may have a choice of what to hold, though in strong turbulence this may no be the case.
Decompression buoys are a relatively common choice in the open sea, for signalling position and for depth control during ascent.
Inshore dive sites in North America with vertical rock faces include Puget Sound in Washington, Monterey Bay, and Catalina Island in California.