Hei, hei, this is a very interesting question that I have asked myself and haven't come to a proper conclusion. Sorry, bummer first. This might be, because I have been to Antarctica, but not as a tourist, so it is hard to say what exactly happens when they come and how that could impact the environment. A quick google revealed this paper (https://www.sciencedirect.com/science/article/pii/S0301479722002079) which also comes to the conclusion that it is hard to estimate. But lets have a look what I can think about.

So the tourism is by ship, because only very few tourists fly into King Georges Island and hang out there. And since this is the base for most expeditions, their impact in addition to the stations, scientists, military, etc. is probably not really relevant. So ships.

Ships are generally impacting marine life by their noise, movements, and especially by the release (voluntary or not) of trash, oils, gas, paint, etc. into the environment. So this will have an impact. Luckily not soo many companies go to Antarctica s of now, so I believe that the impact of the fishing fleet (especially the illegal ones) are larger. And as far as I understand, there are also very strict regulations for these companies and they use very modern ships.

On land, when the tourists walk around, this is usually in small areas and designated landing bays etc.. There, the impact of the tourists directly can be high, but also very local. Birds nest there and might be disturbed, other animals as well, trash can be left behind, people leave their germs (wild toilets...), and many more things. There I can see an impact, although it is very local.

So in my opinion (and this is an opinion, not fact based now) I think it is not good, but also not the worst we are doing to the white continent right now.