No,it gets drained after each use. �Do you use the same water in your
bathtub? �He's not talking hot tub or spa, this is a bathtub.

I still prefer a shower.

many friends with those tubs added to existing homes have undersized
water heaters and thus they cant fill the tub with enough hot water to
use.

since these tubs are generally larger than regular tubs and hold more
water a larger higher capacity water heater is essential.

if your heating by gas a 100 gallon commercial one will likely do the
job.

the trouble with electric is look at the power needed to run one, say
a 100 gallon high capacity.

this issue can require a service upgrade which can be very expensive.
if your using electric to heat water you probably dont have gas.

so combine heating demands with regular home demands and add a large
power hungry hot water tank.