[nonameiguess]

Something like proton decay resulting in the permanent non-existence of baryonic matter is unimaginable timeframes, but the Earth will lose all of its carbon dioxide, and hence all eurkaryotic life, within around 1.3 billion years of now. So that's the longest any multicellular creature inhabiting the ocean can still be around, unless far future tool-users figure out how to migrate to a new planet with chemically similar oceans and give them an assist by taking some.