I don't have any sea world background stamps, but I decided to build corals using CI tree stamps and Distress inks in orange, yellow, pink and violet. I'm actually quite satisfied with the result. The background for this card was made with leftovers from cutting out unmounted stamps: I layed them on an acrylic block with Tack'n Peel (that I use for stamping with the actual stamps too) and to get a soft subtle print I added Distress Inks (first blue, then green) using an ink blending tool. Check here to see what a vibrant look these make with more ink and a spritz of water!
Actually I made a larger scene first, and it's fine enough, but a bit empty (like the life of a sea turtle, I guess...) In this one the bubbly background is a Chocolate Baroque background stamp. To keep it subtle, also here the blue and green inks have been spread with an ink blender. This way the image is light, yet has all the details of the stamp. I especially like the look of the reef on the left (except for that upright part that I covered with the sentiment :) When this was done, I realized the image would easily fit on a 6x6 card too.