Restart=alwaysって書いとくとプロセスが落ちたときに再起動してくれる
sudo systemctlで列挙
/etc/systemd/system/aqua.service
[Unit]
Description=Aqua Service
After=syslog.target
[Service]
Type=simple
WorkingDirectory=/home/pi/AquaServer
ExecStart=/usr/bin/mono /home/pi/AquaServer/AquaServer.exe
TimeoutStopSec=5
StandardOutput=syslog
Restart=always
[Install]
WantedBy = multi-user.target