The SIM provides the link between the underwater IM instruments and the computer / buoy controller. Communication with the computer / buoy controller is via RS-232.
SIM-Direct is for typical cable-to-shore applications, where the inductive modem wire is insulated above the surface and connected to the SIM. A second wire from the SIM connects directly to seawater ground.
SIM-Coupled is useful for buoy applications, where the jacketed wire is grounded to seawater on each end and connected to the SIM via an Inductive Cable Coupler (ICC).