The accessory wire is for connection to an ignition switch. This is what lets the radio turn on when you turn on the key. All you need to do is tie this wire to +12V and the radio will work fine. If you ground it or leave it unconnected the radio will never turn on.
From our competitive research (Viseton Corp, formerly Ford Electronics), it seems the only difference between a marinized stereo and an automotive stereo is the bezel may have a UV coating to keep the sun from destroying it while it sits in the dash of a boat. If your installation is inside a cabin, you don''t need it.