Last time I was in Scapa it was October and it was 12C on the bottom and maybe 13C on stops.
They way you dive it though means you get changed between dives so you would be warm for a bit. Air temperature is never tropical though so there is a good chance you would be cold and miserable putting the wetsuit back on for the second dive.
My longest dive that week was 85 minutes on the Markgraf
SMS Markgraf | Scapa Flow Wrecks including at least 30 minutes of stops. Deciding you are too cold half way through that might not be any fun.
Alternatively, take advantage of the crashing pound and order a suit from these people
Dry Suits Then collect it when you arrive.
Scapa Scuba are brilliant, if you develop a leak on a dive they can take your suit away, fix it over night and have it back on your boat in time to dive it the following day.