Upps, I'm sorry, this was a misunderstanding.... lost in translation... ;-)
No, I don't know any other tour guides from Warnemünde. I guess they either have to book a tour of the cruise company, or do it on their own.
As far as I know most cuise companies only offer tours to Berlin, which I wouldn't recommend. Your parents would spend 6 hours on the road, and see Berlin for only 5 or 6 hours. And unlike Paris or Saint Petersburg Berlin wasn't built to impress at first sight. IMHO these tours are a waste of money.
I've heard some cruise companies also offer tours now which focus on Mecklenburg, i.e. the coastal area around Warnemünde. If so these tours are the much better choice.
Your parents are probably a bit afraid to explore the area on their own, but it's actually very simple. And it will cost only 27€... or even less. Sure, it's impossible to do Warnemünde AND Schwerin AND Wismar if you travel by train, but Warnemünde plus one of the towns is doable. And you have plenty of other options. This here is a good website which explains all the possible day trips from Warnemünde:
https://travelanni.webs.com/
Of course not knowing what you see can be a problem, but you have the Internet and can read up on the history of the place. And the local tourist information usually offers pretty good, compact flyers. But it's not the same of course.
BTW: something probably completely off the radar is the combination Bad Doberan and the Molli Steam Train. Bad Doberan was the summer residence of the Mecklenburg dukes, but the highlight is the minster, one of the best examples of the Brick Gothic in the Baltics. From Bad Doberan they then take the Molli, which crosses the heart of the old town, to Kühlungsborn, a nice seaside resort. There they can relax, have coffee and cake, and return to Warnemünde.
If they just want to see one Hanseatic city I would recommend Stralsund instead of Wismar. Both are World Heritage Sites, but Stralsunds old town has a bit more to offer, like two marine life museums and the Gorch Fock.
BTW: it's also possible to rent a car. There are three rental station on the pier, and they would save quite a bit of time. And driving there isn't difficult, since Mecklenburg is only sparsely populated.