Water usage is growing at more than twice the rate of the population, and an increasing number of regions are reaching the limit at which water services can be sustainably delivered, specifically in arid regions (United Nations Water, 2007). Building large infrastructures such as water transfer systems and seawater desalination plants has gained support to alleviate water scarcity. Seawater reverse osmosis (SWRO) is one of the preferred technologies used in the treatment process of seawater desalination. The quality of the source water plays an important role in extending the membrane life in this system, where it can prevent membrane fouling which occurs because of pore-clogging or adsorption of solute on the membrane surface, which could be a result of the presence of harmful algal blooms. The objectives of this research are to determine the optimum coagulant dose of ferric chloride, ferric sulfate, and ferrous sulfate and the impact of pH on the coagulant dose for removing algae. Also, determining the impact of cationic organic polymer additive, which is polyDADMAC (e.g., Polydiallydimethylammounium Chloride), on floc stability and the minimum economic cost of the coagulants with and without polymer additive. The experiment was done on artificial seawater (33 g/L) containing 1 g/L of bentonite clay and 10 mg/L of sodium alginate to mimic the harmful algal blooms. It was observed that 40 mg/L FeCl3, 20 mg/L FeSO4, or 30 mg/L Fe2(SO4)3 at a pH of 8.25 has the highest turbidity removal, which highly improved the quality of seawater. Moreover, the addition of polyDADMAC to the iron coagulants increased the removal of water turbidity. Furthermore, the coagulation process using iron coagulants led to an increase of more than 90% of total organic carbon, and dissolved organic carbon removal in seawater contains sodium alginate and cultivated algae. When 5.45 mg/L polyDADMAC was added to the coagulants, the removal of total organic carbon and dissolved organic carbon reached more than 75% due to the presence of carbon in polyDADMAC. The iron coagulants and polyDADMAC addition to them have the same performance when tested on artificial seawater containing 10 mg/L cultivated algae instead of sodium alginate where the water turbidity decreases to less than 2 NTU. After the cost analysis was completed, it was found that ferric sulfate without the 5.45 mg/L polyDADMAC has the lowest cost of $0.421/m3 for plant capacity of 1,000 m3/day.