In a medium saucepan, stir together the frozen strawberries, frozen raspberries, water, maple syrup, and vanilla (if using) over medium heat.
Bring to a boil, reduce the heat to medium-low, and simmer, uncovered, for 8 to 12 minutes, or until the berries begin to break down into a sauce, stirring occasionally.
Turn off the heat and use the back of a fork to thoroughly mash any remaining pieces of fruit.
Whisk in the ground chia seed and sea salt, turn the heat on low, and continue to cook for another 1 to 2 minutes, or until it thickens slightly, stirring constantly.
Remove from the heat and transfer to the refrigerator for 5 to 10 minutes to cool slightly. Set aside.