Robert Fisher has kindly pointed out what I missed. The following section appears in the Expert book, page X51:
SWIMMING: All characters may swim unless the DM decides otherwise. Movement rate while swimming is 1/2 normal. The DM will want to decide on the chance of a character drowning when swimming in rough water, swimming while carrying heavy or encumbering equipment or treasure, swimming while wearing heavy armor, or while fighting in the water. The chance of drowning will be largely dependent on the circumstances. For example, a fighter trying to swim in rough choppy seas while wearing plate armor and carrying 3000 cn of treasure might have a 99% chance of drowning. The same fighter in leather armor and not carrying treasure might have only a 10% chance of drowning. When fighting in the water or underwater, it should be harder to hit opponents, and damage done from successful attacks should be reduced. Missile weapons will not normally work while fighting in the water.
Um, thanks Mr. Cook? It’s so overwhelmingly hand-wavey, I almost wonder why he even bothered. Yes, I agree, I will want to decide on the chance of a character drowning. That’s why I’m reading this text. Sigh.