Water depth after ice melts

Answer: the depth remains 4 m; the water level does not change.

Reasoning (Archimedes' principle). Take water density $\rho$. The floating ice displaces a volume $V_d=0.3\ \text{m}^3$ (the part below the surface). By equilibrium, the weight of the ice equals the weight of water displaced:

$$W_{ice}=\rho g V_d=\rho g(0.3)$$

So the ice has a mass of $0.3\rho$. When it melts, it turns into water of the same mass, with volume

$$V_{melt}=\frac{W_{ice}}{\rho g}=0.3\ \text{m}^3$$

The melted water occupies exactly the 0.3 m³ that the submerged part of the ice used to displace, so the total volume of liquid below the original surface is unchanged. The 0.2 m³ of ice above the surface was already part of the ice's weight and is included in the 0.3 m³ of melt water.

$$h_{new}=4\ \text{m}$$

This holds because the ice floats in water of the same kind (pure water). If fresh-water ice melted in denser sea water, the level would rise slightly.