The pressure at the surface level is equal to 1 atm.
For every 10 meters underwater, the pressure increases by 1 atm.
So we can write the following equation relating the pressure P for x meters underwater:
We want the pressure to be 6.48 times the pressure at the surface, that is, 6.48 atm.
So we have:
Therefore the depth underwater is 54.8 meters.