Remove nested if statements

I have been learning a new method of programming. I have been programming many years and am recently finding out there is a better method in general via abstract classes.

My question is how to convert a small section of my code so i can understand how this works entirely. So far i could only figure out how to do 1 part:

public abstract class Water
{
    public abstract string GetString(double AmountToAdd);
}

public class AddTotalWater : Water
{
    public double TotalWater;

    public override string GetString(double AmountToAdd)
    {
        TotalWater += AmountToAdd;
        if (TotalWater > 999999)
            return TotalWater.ToString(Globals.ScientificNotationFormatter);
        else
            return TotalWater.ToString();
    }
}

and use it with:

TotalWater.text = AddTotalWater.GetString(amountToAdd);

I would just like some guidance to learn how to do this fully. Is this the proper way to do this to begin with?

This is the nested nightmare I have currently that i want to convert:

public void DetermineWaterValues()
    {
        if (Time.deltaTime < TotalWaterProductionTime)
        {
            double tempWaterTime = waterCounter += Time.deltaTime;
            if (tempWaterTime > TotalWaterProductionTime)
            {
                tempWaterTime = TotalWaterProductionTime;
                waterCounter = 0;
                WaterProgressBar.fillAmount = 0;
                TotalWater.text = AddTotalWater.GetString(amountToAdd);
            }
            else
            {
                if (TotalWaterProductionTime >= 0.1)
                    WaterProgressBar.fillAmount = (float)(tempWaterTime / TotalWaterProductionTime);
                else
                    WaterProgressBar.fillAmount = 1;
            }
        }
        else
        {
            WaterProgressBar.fillAmount = 1;
            TotalWater.text = AddTotalWater.GetString(amountToAdd);
        }
    }

It's quite complicated to figure out how to do this properly. This is the way i did it for over 10 years heh...