Another trick that involves less hassle and water than the tub is mixing about soap and water (heavy on the soap), then wipe or spray that mixture on the surface. It sticks a bit more than pure water and can show where the leak is via the bubbles.
Depending on the material, I’ve had mixed luck sealing leaks with stuff like heavy duty duct tape or a regular patch kit form a bike tube.
Just to ask the question, how much does one of these balls cost?
Good to compare that against what might need to be a notable amount of sealant to properly coat the internal surface of the ball. I’d have to run some basic shape surface area numbers, but we are probably talking massive increase in areal (10x???). You may not need the total extra like a tire setup for longer term use vs what might seal and be set initially. But I could see this ending up with a decent spend of sealant vs a new ball.