We’ve had a threat or two of a hurricane in our area over the summer and into the fall so more than once we filled up the tubs to store water for flushing the toilets and rinsing things. The largest tub in our place can hold a lot of water but unfortunately it had a slow leak at the drain that over the course of several hours was too severe.
Fortunately I found a very simple solution: electrical tape.
Building a seal around the drain and the drain cap slowed the leak to a point that I couldn’t even tell if it was leaking any more.