In clear sea water, take it that red light falls to half every 2 m, and blue-green light every 12 m…

Step 1. Red light. $\dfrac{8}{2} = 4$ halvings: $100\% \times \left(\tfrac{1}{2}\right)^4 = 6.25\%$.

Step 2. Blue-green light. $\dfrac{24}{12} = 2$ halvings: $100\% \times \left(\tfrac{1}{2}\right)^2 = 25\%$.

Step 3. Why. At $24$ m almost no red light is left, but a quarter of the blue-green light is. Chlorophyll uses red and blue light best. Phycoerythrin absorbs the blue-green light, so red algae still trap enough energy.