Although listed as an "option" by ICOM, virtually every marine SSB installation will require the use of an AT-140 antenna tuner. This is true whether you use a long wire/insulated backstay antenna, or a fiberglass whip - since chances are these antennas won't have an impedance that exactly matches the 50 Ohms required by the M802 transceiver.
Using an actual 50 Ohm matched antenna will not be satisfactory, as all marine frequency bands will not be available; furthermore, the antenna would be huge - hence the antenna tuner.
The antenna tuner electronically matches the impedance of the antenna you use, to the 50 Ohms required by the M802 transceiver.
The operation of the AT-140 is "black box" and completely automatic. All it needs is the proper connections; it can be installed out of the way, but as close to the base of the antenna as possible. On a sailboat, this is usually the lazerette.
The AT-1400 is weather protected, and can be sited in a damp enviroment.