The species depends on when you visited, but in recent years they’ve kept a ringed seal in with the belugas (I saw it this summer).
SeaWorld Orlando has, on occasion, mixed harp seal with their belugas, but it’s very infrequent and I was lucky to see it several years ago. The harp seal is normally kept behind the scenes.
Georgia Aquarium keeps harbor seals with their belugas.
You got to see Natchek the ringed seal? Lucky. I wanted to see him when I visited recently but he was been behind the scenes.