In a large bowl, stir the water and instant yeast until combined.
Add about three-quarters of the bread flour and mix with a spoon until a shaggy dough forms. Add the remaining bread flour and mix until no dry spots remain, 2–3 minutes.
Sprinkle in the fine sea salt and knead on the bowl or a counter until smooth, elastic, and slightly tacky, 8–10 minutes.
Shape the dough into a ball, place in a clean bowl, cover, and let rise at warm room temperature until doubled in size, 60–90 minutes.
Meanwhile, set a baking stone or inverted baking sheet on the middle rack and a sturdy empty metal pan on the lower rack. Preheat the oven to 475°F for at least 30 minutes.
Lightly dust the work surface with the all-purpose flour. Turn out the dough and divide into 8 equal pieces. Shape each piece into a tight ball and rest, uncovered, 10 minutes.
Form marraquetas by arranging the balls in pairs, two balls touching. Press a thin rolling pin or the handle of a wooden spoon firmly along the seam to fuse them, then press once more crosswise to create four connected lobes without cutting through. Transfer the shaped pieces to parchment.
Cover and proof until noticeably puffy and the dough slowly springs back when gently pressed, 45–60 minutes.
Slide the parchment with the loaves onto the preheated stone or sheet. Carefully pour the hot water into the lower pan to create steam and close the oven door immediately.
Bake with steam for 10 minutes, then remove the steam pan. Continue baking 8–12 minutes more, until the marraquetas are deep golden and sound hollow when tapped on the bottom.
Cool on a wire rack at least 20 minutes, then break along the seams into quarters and serve.