1) I don't know the exact number, but I could make a guess. The deeper down you go in the ocean, the more pressure there is because of the weight of the water above you. A pretty simple formula for pressure at different depths of water is:
P = g*r*h. P is pressure, g is acceleration due to gravity (9.8 m/s²), r is the density of the water, and h is the depth.

I don't know how much salt there is in the Santa Barbara channel and I don't know the water temperature, so I don't know its density. For this guess, I'm just going to assume that the density of the water is 1gram/cm³. From what I was able to find online, the maximum depth of the Santa Barbara Channel is about 500 meters. This gives P = (9.8 m/s²)*(1 g/cm³)*(500m)

We have to convert 1 g/cm³ into kg/m³.
1 g = 0.001 kg
1 cm³ = 0.000001 m³

Therefore, 1g/cm³ = 0.001kg/0.000001 m³ = 1000 kg/m³. Plugging this back into the formula gives
P = (9.8 m/s²)*(1000 kg/m³)*(500m) = 4,900,000 kg/(m*s²)
P=49,000,000 N/m² = 49 MPa

2) As water gets warmer, or as its temperature increases, it expands in volume. This is because the water molecules have more energy and bump into each other more, causing the molecules to spread apart. The number of molecules won't change, so neither will the mass. Density is mass divided by volume, however, so when the volume increases and the mass stays the same, the density decreases.

3) A substance's solubility is the amount of it that can be dissolved in a solvent (such as water). This depends on temperature. The density of very salty water is about 1.03-1.04 grams/cm³. I don't know if this is the maximum.

4) Hmm. I think that would depend on a few things. If the rain water is cooler than the pond water, then it would lower the temperature of the pond, making it more dense. Or, if the rain water is warmer, then the opposite would be true. It's also possible that rainwater dilutes the pond water, causing the impurities to spread out and making the overall pond density smaller. But this is just a guess :)

5) I'm not much of an antifreeze expert, either :) Antifreeze lowers the freezing point of water and raises the boiling point. It lowers the freezing point by helping to prevent the water molecules from clustering together to form crystals.

1 - The pressure at the greatest depth in the Santa Barbara Channel (or anybody of water on earth) depends on the depth of the water (and also the density of the water but that changes less drastically than depth). A good rule of thumb I learned from scuba diving is that 10 m of water is equivalent to 1 atmosphere of pressure.

The deepest place in the Santa Barbara Channel is ~270 meters deep, so the pressure is ~27 atmospheres. That means 27 times more pressure than we have on us here at sea level in the Santa Barbara. If you want to get more precise, you would need to also take into account the salt content and temperature of the water, because they affect the density of the water.

2 - At the molecular level, heat is essentially how hard and fast the atoms and molecules are bouncing around. The atoms in a "hot" object are moving more than those in a "cold" object. When the atoms move around more, they bounce off each other and force each other further apart. When the atoms are further apart, the material they are in becomes less dense -- same mass in a bigger volume.

3 - I don't know the answer to this one offhand, but you may be able to conduct experiments about this yourself. You are correct that water is most dense at temperatures a few degrees above freezing. More salt at this temperature will make the water even denser. However, you can't keep adding salt forever. There will be a limit as to how much salt can stay dissolved in the water. You could experiment to see how much salt the water will accept before it cannot dissolve any more. I think you will need a scale, measuring cup, pot, salt and water. I would heat the water (so salt dissolves more easily), then dissolve in as much salt as you can, then cool the water to a little above freezing and weigh some volume of water. 250 ml of pure water should weigh 250g. Can you get it to weigh 300g?

4 - If the pond is fresh water and the rain is fresh water, the density should not change. If the rain and pond are very different temperatures, I guess the density would change a bit, but the effect is really not very noticeable. However, if rain water runs off into the ocean, it is less dense and it "floats" on top of the salt water. This can cause significant local effects in coastal places where it rains a lot (like the west coast of New Zealand).

5 - The most common anti-freeze I know about is glycol. It is a substance with a lower freezing temperature than water, so it is still in liquid form after water has become solid.