I'd like to add some extra context to these images because it breaks my heart. The kids woke up on their last day in Okinawa to do some sight seeing after their mission, and Gojo was already gone for the day. No one had seen him even for breakfast. So they go about their business and they finally see Gojo again at the airport when its time to fly home.
Which means they don't know Gojo is there at the aquarium, and he's purposefully keeping himself apart from them. When its time to go, we see that he bought all of them plenty of souvenirs, even the hotpot set for Miwa that they were disqualified from winning at a beach volleyball tournament (that he also "wasn't there" for). It seems like, while he was legitimately off doing his own thing in Okinawa, which has plenty of angst potential on its own, he was also keeping pretty close tabs on what everyone was up to.
We see Gojo exhibit this behavior of keeping himself separated several times in the Phantom Parade events - in some cases it's so they can "experience their youth to the fullest", or because he thinks he's going to disrupt things by showing up. Either way, there seems to be the belief that his presence will ruin whatever fun is being had and/or put people on edge (in the case of letting the kids heckle Ijichi without joining in for Ijichi's day off event, even though he'd been trying to help Ijichi relax all day).
Gojo probably views this as simple self-awareness. After all, he's a person of authority, The Strongest, and "has a bad personality". We know he isolates himself on purpose from the manga, but Phantom Parade shows us what that can actually look like. And its sad. It's sad that he thinks his very presence would make things worse for the people he cares about. And its sad that for the most part, he's right. He always looks so damn wistful about it too, like he really, really wants to be with them and be a part of things, but just... knows better than to try.
He does turn up during this event and is generally a menace, you guys have probably seen the watergun fight pics, but that's the thing - he shows up when it'd be funny to be annoying, or to bother someone on purpose. Every time he'd like to just be together with others for enjoyment, he holds himself back. And it kills me that, with the exception of Yuuji, Yuuta, and Shoko, he's right to.
Seperately, this event has also provided several "dead wife montage" style pics of Gojo LMAO. The first is the one in the original post, but his SSR beach character's alternate art is just as bad:
(edited post to change the images because the first ones were crunchy as fuck lmao. These are straight from the game's assets)