For configuration or testing purposes, your FloatHub can be powered with the supplied USB cable. For actual installation in your boat, a fused (1-2 Amp) lead should be permanently wired to the bottom two screw terminals as per the installation manual.
Note that while your device may turn on if supplied with power only via a USB cable, USB does not provide sufficient power for reliable permanent operation. In particular, the Cellular versions of FloatHub will not power up the cellular modem at all unless supplied with power via the screw terminals.