Similarities:
- Marine and freshwater ecosystems are water, covering almost 75 percent of the earth's surface.
-The most pervasive problem affecting both freshwater and marine ecosystems is pollution.
-The process of regulating the osmotic pressure on fluids inside an organism is essential to all freshwater and marine species.
Differences:
- Marine ecosystems are more diverse than freshwater ones.
- In marine or seawater, the salinity level, or the concentration of salt is greater, but freshwater is not totally void of it.
- Only 1% of the whole water body on the earth is freshwater, the other 99% is marine water.
Note- I couldn't come up with ones for estuaries, sorry.