The Royal Navy saw significantly less combat during the Last War but still retains an important purpose. While isolated from the brunt of the naval engagements in the Scion’s Sound, the southwestern coast of Breland was important to protect for trading purposes, especially with the government of Riedra. The Royal Navy is furthermore tasked with controlling the waters between Sharn and Xen’drik, as any ship that doesn’t make it back to Sharn is a ship who’s goods are not being taxed by Breland and the Sharn port authorities. While the Sahuagin have signed a treaty not to attack ships passing through their territory, they're not the only threat that lurks beneath the waves.
Source: Politics of Breland