Short answer:

• Shwetambar tradition: the 8th samudra is Vāruṇoda (Varun Ocean). ( jainpedia.org, en.wikipedia.org)
• Digambar tradition: the 8th samudra (in the inner series) is Nandīśvara Samudra. ( encyclopediaofjainism.com)

For a simple overview of Jain cosmology (dvīpa–samudra rings), you can also see this quick explainer on JainKnowledge: