Flow rate problem , filling a tank with gravity flow

Goodday I wonder if anyone can help , I have a problem with an equation where a ships tank floods from sea water ingress .

As the water starts to fill the tank the flow rate is high and deceases as the tank eqalizes with the outside water depth.

I use the eqation q= d^2 * v * pi /4 where

d = diametre of hole in m

v = velocity of water m/s as ( 2*9.8*height)^.5

My problem is how do i plot the on an x and y to find the f'(x) where i can see at any point in the tank becoming full , what will the flow rate be, in m^3 /sec

thanks

rob