The excursion will drop you off at the top of the baths and you would have to take the trail down to the beach. It is over rocks, with irregular steps, and could be challenging if mobility and knees are an issue.
Here are some images from the restaurant area - you will see the restaurant, deck, and views. It would be a comfortable place to wait, and you can get a sense of the size of the boulders. We saw a tour group coming through that had some mobility issues, and I remember thinking it was going to be a miserable hike down for some of those folks. And then of course, you have to hike back up hill.
There is
an alternate path which takes you to the other side of the traditional baths beach rocks, to Devil's Bay. So you can enjoy Devil's Bay, then climb through the boulders (mobility would be an issue here as there are tight spaces and bendy moves required) on a marked trail, to the busier side of the baths, and then climb out via the highly-trafficked path (making a circle instead of an out-and-back). To access the path to Devil's Bay, you take a left at the top of the hill whereas the other path goes right (and by the restaurant, shops). The path to Devil's Bay is actually easier to navigate - still with some steps and dealing with rocks but not as steep as the main path. We liked doing the circle. There were only a four of us on the beach in Devil's Bay - most people climb through the rocks to Devil's Bay and then back to the other beach. After climbing through to the main beach we enjoyed a snack (hamburger, pop) at the snack stand. All the paths are dirt, rocky, and uneven, so you must judge your mobility.
Alternatively, we noticed some private boat charters who backed up to the area, and visitors disembarked in the water to the beach instead of coming down from the top. Swimming skills recommended if you go that route.
And yes, the ferry ride from the dock to the island where the baths are will be choppy in areas so don't be surprised at that - the crossing currents create chop.