Cross-lighting is an underwater photography technique that enhances textures and contrasts in your subject by placing your strobes across from each other, with the subject in-between. This method is great for capturing details in static subjects, like sponges and fish, as it helps eliminate distractions from the background. Long strobe arms are important for positioning the lights correctly, especially in macro photography where you want to show intricate details. Overall, using cross-lighting can dramatically improve your underwater images by allowing your subjects to stand out.