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The density of lead is more than 11 times higher than the density of water. You will need the water density to increase due to pressure at the bottom by a similar factor. The pressure at the deepest ocean areas (about 11 km) is about $ 11 \times 10^7 Pa $. The bulk modulus of water being about 2 GPa, the change in volume produced by a pressure of $ 11 \times 10^7 Pa $. will be about $$ \frac{11\times 10^7 Pa}{2 \times 10^9 Pa} = 5.5 \times 10^{-2} $$ or about 5%. This is the order of magnitude of the density changes in the water. The effect is negligible compared with the goal. And compresibility of water at high pressures may be even higher that the value used in this estimation so the effect will be even smaller.

If you want some imaginary situation rather than realistic depths, we could estimate the pressure for water to reach density of lead. Assuming a still linear behavior of water pressure with depth, to have an increase in density of 11 times rather than 0.05, the depth should be about 11/0.05=220 times more than the 11 km. So about a couple a kilometers. However at this depth the gravity is significantly reduced (Radius of Earth 6400 km). The water compresibility may change significantly. And it way even suffer a phase transition. This should be looked up.
Yes, looking at phase diagrams of water, at pressures over about 10 GPa it will be in one of the solid phases so the question of buoyancy becomes irrelevant.

So the final point I think is that water will be solid before its density in liquid form becomes equal to the density of lead.

The density of lead is more than 11 times higher than the density of water. You will need the water density to increase due to pressure at the bottom by a similar factor. The pressure at the deepest ocean areas (about 11 km) is about $ 11 \times 10^7 Pa $. The bulk modulus of water being about 2 GPa, the change in volume produced by a pressure of $ 11 \times 10^7 Pa $. will be about $$ \frac{11\times 10^7 Pa}{2 \times 10^9 Pa} = 5.5 \times 10^{-2} $$ or about 5%. This is the order of magnitude of the density changes in the water. The effect is negligible compared with the goal. And compresibility of water at high pressures may be even higher that the value used in this estimation so the effect will be even smaller.

If you want some imaginary situation rather than realistic depths, we could estimate the pressure for water to reach density of lead. Assuming a still linear behavior of water pressure with depth, to have an increase in density of 11 times rather than 0.05, the depth should be about 11/0.05=220 times more than the 11 km. So about a couple thousand kilometers. However at this depth the gravity is significantly reduced (Radius of Earth 6400 km). The water compresibility may change significantly. And it way even suffer a phase transition. This should be looked up.
Yes, looking at phase diagrams of water, at pressures over about 10 GPa it will be in one of the solid phases so the question of buoyancy becomes irrelevant.

So the final point I think is that water will be solid before its density in liquid form becomes equal to the density of lead.

The density of lead is more than 11 times higher than the density of water. You will need the water density to increase due to pressure at the bottom by a similar factor. The pressure at the deepest ocean areas (about 11 km) is about $ 11 \times 10^7 Pa $. The bulk modulus of water being about 2 GPa, the change in volume produced by a pressure of $ 11 \times 10^7 Pa $. will be about $$ \frac{11\times 10^7 Pa}{2 \times 10^9 Pa} = 5.5 \times 10^{-2} $$ or about 5%. This is the order of magnitude of the density changes in the water. The effect is negligible compared with the goal. And compresibility of water at high pressures may be even higher that the value used in this estimation so the effect will be even smaller.

If you want some imaginary situation rather than realistic depths, we could estimate the pressure for water to reach density of lead. Assuming a still linear behavior of water pressure with depth, to have an increase in density of 11 times rather than 0.05, the depth should be about 11/0.05=220 times more than the 11 km. So about a couple a kilometers. However at this depth the gravity is significantly reduced (Radius of Earth 6400 km). The water compresibility may change significantly. And it way even suffer a phase transition. This should be looked up.
Yes, looking at phase diagrams of water, at pressures over about 10 GPa it will be in one of the solid phases so the question of buoyancy becomes irrelevant.

The density of lead is more than 11 times higher than the density of water. You will need the water density to increase due to pressure at the bottom by a similar factor. The pressure at the deepest ocean areas (about 11 km) is about $ 11 \times 10^7 Pa $. The bulk modulus of water being about 2 GPa, the change in volume produced by a pressure of $ 11 \times 10^7 Pa $. will be about $$ \frac{11\times 10^7 Pa}{2 \times 10^9 Pa} = 5.5 \times 10^{-2} $$ or about 5%. This is the order of magnitude of the density changes in the water. The effect is negligible compared with the goal. And compresibility of water at high pressures may be even higher that the value used in this estimation so the effect will be even smaller.

If you want some imaginary situation rather than realistic depths, we could estimate the pressure for water to reach density of lead. Assuming a still linear behavior of water pressure with depth, to have an increase in density of 11 times rather than 0.05, the depth should be about 11/0.05=220 times more than the 11 km. So about a couple a kilometers. However at this depth the gravity is significantly reduced (Radius of Earth 6400 km). The water compresibility may change significantly. And it way even suffer a phase transition. This should be looked up.
Yes, looking at phase diagrams of water, at pressures over about 10 GPa it will be in one of the solid phases so the question of buoyancy becomes irrelevant.

So the final point I think is that water will be solid before its density in liquid form becomes equal to the density of lead.

The density of lead is more than 11 times higher than the density of water. You will need the water density to increase due to pressure at the bottom by a similar factor. The pressure at the deepest ocean areas (about 11 km) is about $ 11 \times 10^7 Pa $. The bulk modulus of water being about 2 GPa, the change in volume produced by a pressure of $ 11 \times 10^7 Pa $. will be about $$ \frac{11\times 10^7 Pa}{2 \times 10^9 Pa} = 5.5 \times 10^{-2} $$ or about 5%. This is the order of magnitude of the density changes in the water. The effect is negligible compared with the goal. And compresibility of water at high pressures may be even higher that the value used in this estimation so the effect will be even smaller.

The density of lead is more than 11 times higher than the density of water. You will need the water density to increase due to pressure at the bottom by a similar factor. The pressure at the deepest ocean areas (about 11 km) is about $ 11 \times 10^7 Pa $. The bulk modulus of water being about 2 GPa, the change in volume produced by a pressure of $ 11 \times 10^7 Pa $. will be about $$ \frac{11\times 10^7 Pa}{2 \times 10^9 Pa} = 5.5 \times 10^{-2} $$ or about 5%. This is the order of magnitude of the density changes in the water. The effect is negligible compared with the goal. And compresibility of water at high pressures may be even higher that the value used in this estimation so the effect will be even smaller.

If you want some imaginary situation rather than realistic depths, we could estimate the pressure for water to reach density of lead. Assuming a still linear behavior of water pressure with depth, to have an increase in density of 11 times rather than 0.05, the depth should be about 11/0.05=220 times more than the 11 km. So about a couple a kilometers. However at this depth the gravity is significantly reduced (Radius of Earth 6400 km). The water compresibility may change significantly. And it way even suffer a phase transition. This should be looked up.
Yes, looking at phase diagrams of water, at pressures over about 10 GPa it will be in one of the solid phases so the question of buoyancy becomes irrelevant.