Is there a boat tour from Marmaris to see the rock tombs - that doesn't take very long - and doesn't include lunch and the mud baths etc. we are in Marmaris only one day on a cruise - and are seniors can not sure we could handle an entire day on one of the advertised boat excursions in the port.
Pick out a Lonely Planet Turkey book and see if there are such tours, although if they are all-day tours, they'll very likely include lunch. If the mud baths are on the way, they probably won't skip them either unless they have short versions of the tour. I believe the area is called Daylan.
You don't have to be on a boat to see the Lycian tombs. They are visible from many restaurants along the river in Dalyan. If you're interested in sea turtles, there is an interesting sea turtle rehabilitation hospital at Iztuzu beach in Dalyan that you might be able to visit as well. If your ship does not offer a trip to Dalyan that does not include the boat tour, perhaps you and another couple could hire a taxi in Marmaris to take you to Dalyan for a couple of hours? You might be able to arrange this on the Roll Call section of Cruise Critic. If you do decide to take the boat trip, I can tell you that the time goes quickly and we enjoyed it. We're seniors (67 & 70) and were there last month. We didn't intend to do the mud bath but decided to at the last minute, and it was quite the experience! I'd suggest wearing a swimsuit under your clothes in case you get tempted too.