Simple Encounter tables for Underwater terrain for Low (1-4),
Mid (5-10) and High (11+) levels. Support for Volo's Guide to Monsters and Mordenkainen's Tome of Foes.
Extra tables for underwater locations, beasts, humanoids, hazards, sahuagin and sea spirits.
Tables use 2-20 range designed with unusual and dangerous encounters occurring less often using 1d12+1d8 or 2d10. Or use 1d20 and ignore this.