They are fungus-like organisms calle oomycetes. Unless you are using a microscope you won't see them - they are microscopic organisms. They are called 'water molds' because of their preference for moisture and high humidity and ability to survive and move with running water. Some of the most devastating of plant pathogens are associated with water molds - potato and tomato late blights, sudden oak death, various root rots, pythium (damping off). Downy mildew is also water mold and its presence is typically quite visual compared to others and usually less fatal - one generally only sees the end result of other water molds, which is plant death.

So with effective drainage and not leaving your plant's leaves wet, it's not much of a concern?

Have a look at this.

Here is a link that might be useful: Diagnosis and Control of Phytophthora Diseases