From the many cruises we've been on, and the many tours we've booked and taken from them, I'm not sure if there are any "general tipping guidelines" for these. I will note this. On "tips included" cruise lines, the brochures state that tipping for exurxions is NOT included. Yet, I've yet to see a "tip jar" on any excursion booked through one of these lines. On on lines that have "envelope" or "automatic" tipping, we have seen "tip jars." I have noted that $5 per couple seems to be a bit of the norm. So we just did what everyone else was doing, if we liked he excursion.
There can't really be any "general guidelines" on this subject, as tipping customs vary al over the world. Our US land standard of 15% to 20% is probably the HIGHEST in the world! In marked contrast, tipping is NOT customary is Austrailia, and some service workers there are insulted if you offer them one. When you are on an excursion or a tour from a cruise, the tipping of the tour or excurision guide will depend upon the LOCAL economy and how tips are regarded there.
For privately booked tours, here is an example. We and another couple booked a limosine for a tour of Rome. The tour was nine hours long, and showed us virtually all the sites, while avoiding the times with traffic tie-ups and lines and crowds at the sites. Each of the four of us tipped 10%, and the limo driver seemed quite pleased with that.