What is the pressure of the water in pascals at the bottom og a lake which is 5 meters deep?

1 answer

water density, rho, is about 1,000 kg/m^3

the mass of a column of water one square meter in cross section and h meters high is therefore:

mass = rho * 1 * h
= 1,000 h kilograms

the weight of that mass is mg =
1,000 * 9.81 * h Newtons

the pressure is that force over the one square meter where Newtons/m^2 is called Pascals

9,810 h Pascals

Now you should really distinguish between "gage" pressure and "absolute" pressure. Gage pressure is just what we said here:
9,810 * 5 Pascals
if you want absolute pressure you must at one atmosphere to that which is about an additional 10^5 Pascals