if I had a photo of a Dolomedes, referring to it as Dolomedes sp. would be incorrect?
No, that would be fine, as long as you were certain it was a Dolomedes.
You’ll see that often in field collection notes.
Nacaduba sp. 1
Nacaduba sp. 2
sp. 1 and sp. 2 to indicate that it was clearly two different species of Nacaduba that were seen, but the observer was unable to identify them to species at the time.
This sort of notation is important for species counts. Without the 1 and 2 notation a different person going over the data later might not know that it was 2 different species seen and count them both as one species (2 observations) for the purposes of species counts.