All depends of what you want.  In general, you want a rich media with nutrients in salty water.  If you have access to seawater, just use it with nutrient agar, yeast extract or any non-specific media.  If you don´t have seawater, you also have to add different salts and nutrients.  I usually used Glycerol Artificial Sea Water Agar (Smith & Hayasaka, 1982. Nitrogenase Activity associtaed with Halodule wrightii roots. App. Env. Microb. 43(6): 1244-1248)

I isolate marine organisms in the 10-200 um size range that I have collected using a 20 um plankton net. I then use a drawn mouth pipet (made by heating the tip of a Pasteur pipet and gently drawing it out to a fine point w/ forceps, then break the tip to the appropriately-sized opening) attached to tubing which is then used to suck the organisms up. The organisms are then placed in cell wells with filtered seawater (FSW) of the same salinity as the water they were collected from. If the organisms are heterotrophic, add food such as Isochrysis or Rhodomonas, then incubate for several days. Sometimes you have to try different foods to get them to grow. If the organism is an autotroph, then use F/2 medium instead of FSW. Good luck!

You can use following samples to isolate the microbes.

1. Sea water

2. Marine algae

3. Corals and sponges

4. Marine fish and other organisms

First you can culture them in sea water agar plates. It can be made with bacto agar + sea water. Then if you use sea water around 100 micro liter of sample spread on the plate using sterile glass beads. Then Incubate them at 30 0C for 16 hours.

After 16 hours you can identify different shapes of colonies. But they may not have prominent colors. Then pick that single colonies and strike separately on marine agar plates. Incubate with same conditions mentioned above. Then you can see the colorful colonies. If you need further separation... separate them again with picking the single colonies until contamination will be not found. If you need to identify them, simply use the colony morphology or you can do the 16s rRNA sequencing analysis.

** When you separate the colonies, make sure your own labeling method to identify each colony. continue that labeling method until fully identify the bacteria or fungi.

Best regards.