Weigh two 1-quart containers. Fill one with tap water and the other with an equal amount of ocean water. (If ocean water is not available, dissolve 1 ounce of table salt into a second quart of water.) Now weigh the containers again.

By subtracting the weight of the empty container from the weight of the container with water in it, you will find out how much each volume of water weighs.

Similarly, two large balloons can be attached to the ends of two 10 foot lengths of rubber tubing. Fill one tube with tap water and the other with ocean water (real or simulated). Hold the balloons closed so that no water enters them.

When each tube is filled to capacity, allow the water to flow into the balloons. The balloon filled with ocean water will expand more than the balloon filled with tap water.

Why?

Because of the weight difference from the dissolved salt and other materials, water pressure is greater 10 feet deep in ocean water than it is 10 feet deep in fresh water.

Does the weight of water have anything to do with the water pressure felt in the depths?

How would you weigh the ocean? How would you figure out how much water is in it? Which ocean? Any of them.