Mason notices that his boat sinks lower into the water in a freshwater lake than in the ocean. What could explain this?

Ocean water has more salt than freshwater. More salt causes you to float. For example, the Dead Sea is so salty that it is nearly impossible to swim, you always float while you're in it due to all the salt.

Salt water is more dense than freshwater.

Step-by-step explanation:

A boat is always able to float on water because it experiences an upthrust upwards.

The magnitude of the upthrust depends upon the density of the liquid.

When the liquid is denser the boat will experience a high upthrust as compared to when in a less dense liquid.

Now, ocean water consists of salt which makes it more dense, thus If the boat sinks lower into the water in a freshwater lake than in ocean, then Salt water is more dense than freshwater.