Prepare the Dough: In a large mixing bowl, combine the warm water and sugar. Sprinkle the yeast on top and let it sit for 5 minutes until frothy. Add the melted butter, salt, and flour to the yeast mixture. Mix until the dough comes together, then knead for about 5 minutes on a floured surface until smooth and elastic. Place the dough in a greased bowl, cover, and let it rise for about 1 hour, or until doubled in size.

Shape the Pretzels: Preheat your oven to 450°F (232°C). Line two baking sheets with parchment paper and set aside. Punch down the dough and divide it into 16 equal pieces. Flatten each piece into a rectangle, place a piece of mozzarella in the center, and wrap the dough around it, pinching the edges to seal. Roll the stuffed dough into a log and shape it into a pretzel by forming a U, crossing the ends, and pressing them down onto the bottom of the loop.

Boil the Pretzels: In a large pot, bring the 10 cups of water and baking soda to a boil. Drop the pretzels into the boiling water, one or two at a time, for 30 seconds. Use a slotted spoon to remove them and place them on the prepared baking sheets.

Bake the Pretzels: Brush the boiled pretzels with melted butter and sprinkle them with coarse sea salt. Bake for 12–15 minutes, or until golden brown.

Serve: Brush the warm pretzels with more melted butter for extra flavor. Serve immediately with marinara sauce for dipping, if desired.