You need to have a 1st class ticket or a 2nd class ticket with an upgrade for 1st class in order to ride the Gotthard Panorama Express. The GoPEx consists of 1st class coaches only.
The boat is leaving from Flüelen at noon (12:00). You can find the schedule here: https://www.sbb.ch/en/leisure-holidays/trains-trips/rail-travel-specialtrains/panorama-trips/gotthard-panorama-express/timetable.html
There is a guide onboard the GoPEx, she will guide you to the boat in Flüelen which is awaiting you just opposite of the railway station.
Since you have a 1st class ticket, there isn’t a space issue in the boat. 1st class is on the boat’s upperdeck and usually very quiet. I recommend everyone to travel 1st class on the lake steamer, it’s an unforgettable experience.
Have a good trip!