A character wielding a spiked chain can attack underwater without penalty, unlike a character wielding a slashing/bludgeoning weapon. Those poor sods take a -2 penalty on attack rolls, and deal only half damage. That makes sense... no?
Source
SRD - Combat Adjustments Underwater
Other great tidbits: a creature
with a swim speed can specifically make
tail attacks without penalty, but takes the normal -2 attack bonus / half damage penalty on other slashing/bludgeoning melee attacks.
However, a creature
without a swim speed takes a -2 attack roll / half damage penalty with all slashing/bludgeoning attacks - except while grappling, when the -2 attack roll penalty
does apply, but the half damage penalty does
not.
As an aside, the superscript ¹ must apply generally and not just to creature without a swim speed that succeed on a swim check and are attacking with a slashing or bludgeoning weapon which is not a tail... that's just got to be an unfortunate layout, right?