Our cruise ship docks at 8am in Cairns.
How far to where the snorkeling excursions leave @1 Spence Street/Fleet Reef Terminal?
Is it walkable for a not so young couple? it's an 8:30 check in.

Check and see where your ship docks. From our experiences, smaller cruise ships can dock in Cairns, walking distance to the small boats harbour but larger ships tender passengers into Yorkiy's Knob, about 30 minutes drive from central Cairns.

It does depend on where your ship docks.

You'll likely struggle to make an 8.30am start, wherever it docks. Check the finishing time of the tours against the ship's planned departure also.

It may be better to do the ship's excursions. At lest then you know the ship will wait for you.

If your cruise ship docks right in Cairns, it would be about ten minutes walk to the area where the reef trip boats leave from. We were there recently to get a ferry to Fitzroy Island and it was about another five minutes along the marina to the right berth, it was fairly long. If they will let you on the boat right up until 8.30, you might make it. I'd tell them you are coming off the cruise ship. If it docks at Yorkeys Knob, it won't work.