There should be no problem using the AWAC in 5 meters or less. The AST measurement operates to ranges as little as 0.7 meters. Below this depth the pressure sensor can be used for non directional wave measurements.  Depths that are less than 2 meters may present challenges for directional wave measurements, since the velocity cells will get interference with the surface. Current profile measurements should not be a problem as long as the cells sizes are maintained small. Again depths that are very shallow will have challenges for current measurements.  

The optimal settings for the AWACs are as the ranges in the deployment software (Estimated depth selection):

1 MHz: 2 m
600 kHz: 5 m
400 kHz: 20 m

Shallow water environments are difficult, but you could get useable AST data more shallow than the values listed above. The absolute minimums are:

1 MHz: 0.7 m
600 kHz: 0.95 m
400 kHz: ~2 m

The minimum distances are based on the start of the AST cell. 

You can consult the Principles of Operation manual to better understand the measurement cell positions and position relative to the surface.