Ferrocement Bath Tub

Hey not sure how to navigate writing a post still. Im looking for feedback on using ferrocement for a bathtub and if anyone has experience filling them with an on demand hot water heater... ive made water tanks but not a bath tub and am worried the hot water heater wont be enough to heat up the tub....

Cool idea.
Indoor or out?
I have built a kitchen counter top out of wood scraps, with nylon window screen stapled to it.
I finished in fiber cement and hydro stop cement.
It's held up very well with constant moisture, but its never been underwater.
I recommend wood to support  a wire and mesh form, because its cheaper and easier to work with and it can easily be finished in water resistant materials for longevity.