In order to complete this star, you will need to have completed Bowser in the Fire Sea, as that will remove the submarine in the second area and some moving poles will spawn. Get to the second area and hop onto the boardwalk, where you want to find the purple switch. This causes two sets of blocks to appear, acting as stairs. The one near the switch lead to the submarine when it was still there, while the one a little bit away will lead to a platform with Red Coin #1. Once you collect the first coin, you will have two ways to go, either to the distance or to the side.
Jump on the pole and collect the three Red Coins (#2, #3, and #4) along the way, making your total four now. Return to the first platform and take the other pole this time, to a platform with the shadow of the star. Again, you have two options here, either to go deeper in the same direction or to the side once more. Once again, take the side path, knowing that you will have to transfer poles twice before you can reach a platform with the Blue Coin Block. From there, you will have to jump on one more pole to reach Red Coin #5. Make your way back to the platform with the star’s shadow, then take the other pole.
When the pole reaches its end, jump to the other pole moving sideways and go to its end to collect Red Coin #6. You will want to stay on this pole and wait for the correct timing to jump to the pole on the opposite side of the metal grates. A good point in time for the jump is when the poles are moving in opposite directions. Getting on this final pole will allow you to collect Red Coin #7 and Red Coin #8. Retreat back to the one platform and collect the star to finish.