SHow him some stats. Guys like numbers usually. Here's a start :
"While parents might think they will only be out of the room for a minute while they fetch a towel or a change of clothes, this absence can be all the time required for a toddler to drown" said Rob Bradley CEO, Royal Life Saving. Baby bath aids are also no substitute for adult supervision they are only designed to assist an adult bathing a child."
Bathtubs – Next to swimming pools, bathtubs pose the greatest home drowning risk. In 2002, the CPSC reported 69 cases of bathtub drownings in children. 80% of them were under age 2. Being with an older sibling does not substitute for parental supervision: almost 40% of bathtub drownings occurred when another child was in the tub.