Movement

Characters moving about underwater have to swim. See Swimming under Chapter 14: Time and Movement in the Player’s Handbook.

Characters who are only wading are much less affected. Wading in knee-deep water reduces movement by 1/3. Wading in waist-deep water reduces movement by 1/2 and adds a one-phase initiative penalty to any action the character takes. Wading in shoulder-deep water reduces movement by 2/3, adds a two-phase penalty, and causes the character to lose any Dexterity adjustment to his Armor Class (although it does provide cover against someone firing missiles at the character).

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