We always get to the terminal as early as possible, based upon what time the cruise line begins processing their passengers. In that way we get to board the ship as soon as they commence boarding; check out our cabin; have some lunch; explore the vessel somewhat; speak with the maitre d' if needed; and get prepared for the fire drill.
There's a lot to do on the ship prior to sailing, and we recommend arriving early to get right into the swing of things. You've paid your fare, so why not enjoy all facets of the cruising experience? For us, that experience begins hours before the vessel leaves the dock.