Yes, all machines in a multimachine deployment must be upgraded to the same version of ArcGIS GIS Server and the Maritime server extension.

No, if you already have Maritime server installed, the Maritime server extension setup will automatically upgrade it to ArcGIS Maritime 11.5. After the upgrade, you will only see the ArcGIS Maritime 11.5 server extension in your list of programs.

You do not need to back up anything before the first installation of ArcGIS Maritime. If you are upgrading Maritime server, refer to Upgrade Maritime Chart Service for details.

No, the setup restarts ArcGIS Server after the installation.

Yes, the version of the Maritime server extension must match ArcGIS Server. You cannot run mismatched versions of software. When you upgrade, you will no longer see Mapping and Charting Solutions; the ArcGIS Maritime server extension will appear in your programs list instead.