Sumi ink works well in seawater. Not all black inks can be substituted. India ink will not work. Sumi ink is the ink used in Asian calligraphy and is available in art supply stores. Unlike the motile sperm, a dead egg can look the same as a live one.
Sea urchins are marine creatures.
The composition of sea water is very different from that of fresh water. Most organisms that live in one of these environments cannot live in the other. Sea water is hypertonic, tap water is isotonic and distilled water is hypotonic to spring water. This would be a hypertonic solution in regards to sea water, where the fresh water would be a hypotonic solution. Determining size of each of the gametes is certainly a good math problem.