Tub refers to a ship. I think you should watch the film. It stars Nicholas Cage so I am certainly not going to do it. By the way, it is called 'Lords of war'. The reference to 'weight class' means another ship of similar size/weight.

I think this is what it's talking about: In a bathtub, people sometimes install handles (bars) on the wall so that they can hold onto the handle to keep their balance. The handle would need to be strong enough to support the full weight of someone, so heavier people might need a stronger handle, one that will support their weight.

"Weight class" refers to a defined span of included weights. For instance, in the sport of wrestling participants spar with others of their own weight class so that the event is evenly matched. The weight classes may include wrestlers up to 106 pounds, over 106 but less than 113 pounds, over 113 but under 120, and so on. As to the handle of the tub, your narrative does not provide enough information for an answer.