Don’t know what a bathtub water bob is.
A very useful emergency prep water storage containercan be a kids inflatable pool.
example of common size:
Intex 8.5ft x 5.75ft x 22in 198 Gallon Inflatable Family Swimming Pool.( about $60.
smaller sizes come in very compact boxes easiley stored in vehicles in bug out scenarios...
For shelter in place garages, basements etc are good spots to place them. Truck beds as well. Indoor placement safeguards against leaves, bugs etc. as well as being hidden from casual observation by goblins. add hoses, car type tire inflators (make sure you compatiple nozzles. Foot bellows as backup.
90 gal capacity can usually be had for less than $30.
Forgot to add that published fill capacity is based on folks in the pool...for storage purposes pools can be filled to brim, adding substantially greater capacity.