The closed sea otter exhibit was in the Oceanium in the area at the end that also has viewing of the sea lions, the kelp tank and some small invertebrate tanks. I believe the old exhibit, now empty, might have been entirely or partially boarded up now though. For a while it contained some trout but I don't think you can see all of it now.
This exhibit I believe housed Eurasian otters until the pair died of old age in 2017 or 2018. This exhibit, however, was originally constructed to house Chinese alligators, but those passed away during their first hibernation in this exhibit and were never on show here. Picture below confirms it was an otter exhibit two years ago. European otter enclosure - ZooChat