This study aims to investigate the abundance, community, and structure of phytoplankton, physicochemical parameters, and some eutrophication state indices, to estimate the water quality of eight selected beaches along the Alexandria Coast, in the southeast of the Mediterranean Sea. The samples were collected monthly from 2019 to 2020. Nutrient values ranged from 1.54 to 33.21 µM for nitrate, 0.01 to 1.98 µM for nitrite, 0.12 to 9.45 µM for ammonia, 0.01 to 1.54 µM for phosphate, and 0.67 to 29.53 µM for silicate. Phytoplankton biomass was characterized by chlorophyll-a concentration, which fluctuated between 0.12 and 12.31 µg L−1. The annual phytoplankton average was 63.85 ± 17.83 × 103 cells L−1. Phytoplankton was highly diversified (228 taxa), and the most diversified group was diatoms (136 taxa), followed by a remarkably low number of Dinophyta (36 taxa). Diatoms reached maximum abundance in December. Meanwhile, a dense bloom of microalga Chlorella marina occurred in June on some beaches. High temperature, high dissolved inorganic nitrogen, and less-saline waters have supported green algal proliferation. The Shannon–Wiener diversity index (H’) showed that there was a qualitative seasonal difference in the composition of the phytoplankton community. Waters of beaches 1–3 were classified as between clean and moderately polluted; and beaches 4–8 varied between moderately and heavily polluted. The study revealed that human activities might have triggered the algal bloom and may be responsible for alterations in the Alexandria coast ecosystem.