The specific magic weapon rapid wrath (Ghostwalk Campaign Setting 66) (11,702 gp; 3 lbs.), in part, says, "This +1 mighty cleaving shortspear doubles the speed of a creature that carries it" (link mine).
The crystal of aquatic action (lesser) (Magic Item Compendium 25) (1,000 gp; 0 lbs.), in part, says that when it's affixed to your armor you "gain a Swim [sic] speed equal to one-half your land speed."
When an adventurer possesses rapid wrath and affixes to his armor a crystal of aquatic action (lesser), is the adventurer's swim speed his land speed or half his land speed?
That is, does the adventurer, essentially, benefit from rapid wrath twice if the adventurer is lucky enough to have another movement mode's speed based on his land speed? Or is the adventurer already benefiting from rapid wrath because the adventurer's land speed was doubled to compute the new movement mode's speed, and his swim speed remains half his land speed?
Note: As I type this, I (the DM) am leaning strongly toward the former—that is, doubling the newly acquired swim speed—, but the conscientious player whose PC carries rapid wrath and just acquired the crystal of aquatic action (lesser) thinks this is double-dipping and remains curious as to the community's opinion.