Both types of walls have the same purpose of protecting your property and retaining the soil behind them. Seawalls have the added factor of blocking large masses of water from stealing your land, causing soil erosion.
All the “water weight” applied to your retaining wall or seawall can cause them to bow, buckle, lean, or even crack if enough pressure is applied. “Water weight” is the top contributor to retaining wall or seawall failure. You don’t want this, especially when it means you’ll have to replace them. Replacing a seawall or retaining wall is generally more expensive than repairing it.