Oceanus!

Oceanus is seedworld on the opposite side of the Milky Way galaxy from us, where the only ecosystem is the ocean! The seafloor is mostly made up of gravel, though it's also made up of sand, dirt, and clay. It can rain on this planet, and the global temperature is ~57 degrees Fahrenheit. It also has a moderately humid climate with normal rainfall events.

Now, I have some very specific help (because I have to). What I wish to accomplish is successfully seeding a world of ocean. However, I do not know what species (other than eelgrass (Zostera marina) and oarweed (Laminaria digitata)) to seed it with. Once you guys help me with finding out what to seed it with, I will start the process. In other words, I would like help with seeding the wonderful world of Oceanus!