Any aquatic invertebrate could be described that way. Perhaps the term "marine invertebrate" is what you are looking for.

Invertebrate sea life includes the following phyla:

• Acoela;
• Annelida, (polychaetes and sea leeches);
• Brachiopoda;
• Bryozoa, also known as moss animals or sea mats;
• Chaetognatha;
• Cephalochordata;
• Cnidaria, such as jellyfish, sea anemones, and corals;
• Crustacea, including lobsters, crabs, shrimp, crayfish, barnacles, hermit crabs, mantis shrimps, and copepods;
• Ctenophora, also known as comb jellies;
• Echinodermata, including sea stars, brittle stars, sea urchins, sand dollars, sea cucumbers, crinoids, and sea daisies;
• Echiura;
• Gnathostomulids;
• Gastrotricha;
• Hemichordata;
• Kamptozoa;
• Kinorhyncha;
• Loricifera;
• Merostomata; also know known as horseshoe crabs;
• Mollusca, including shellfish, squid, octopus, whelks, Nautilus, cuttlefish, nudibranchs, scallops, sea snails, Aplacophora, Caudofoveata, Monoplacophora, Polyplacophora, and Scaphopoda;
• Myzostomida;
• Nemertinea (ribbon worms);
• Orthonectida;
• Phoronida;
• Placozoa;
• Porifera (sponges);
• Priapulida;
• Pycnogonida (sea spiders);
• Sipunculida;
• Tunicata, also known as sea squirts;
• Some flatworms of the classes Turbellaria and Monogenea;
• Xenoturbella;
• Xiphosura.

I notice that this was originally posted in "aquatic mammals". All mammals are vertebrates so none of the aquatic mammals would lack a backbone.