Don't ya just hate it when that happens?
Sea urchin punctures.
In some dive venues, such as Cocos Island, you can pretty much count on an urchin run-in. Generally with such punctures there is initial intense pain that gradually diminishes over the course of a week or so with ultimate complete resolution. After tweezing out protruding spines, any remaining fragments typically will disappear through a combination of being dissolved and being encapsulated by fibrous material and forced to the surface of the skin. This can take months and there is nothing to be done to hasten the process short of surgical removal. The most important step over the healing period is to monitor carefully for infection. Be aware that urchin spines often have a dark pigment on the surface that remains in the wound after the spine is removed. This pigment does not cause pain and swelling, and should not be confused with actual spines and probed at.
Spines that have become embedded over bony prominences, within joints, or near nerves may result in complications. For example, significant swelling and pain, and even impaired strength and dexterity in areas like the hands and feet, can persist for many weeks. In these instances, surgical removal may be advised.
Immediately after urchin puncture, remove any spine fragments that you can reach. Tweezers can be used to snare protruding ends. Then carefully shave the area with a razor, which also can help in removing barbs. Next scrub the area thoroughly with soap and fresh water, followed by copious flushing with fresh water.
The affected area(s) should be immersed in hot water (as hot as can be tolerated without burning the skin) for at least an hour and preferably more as this greatly reduces pain and swelling. Immersion can be repeated if pain recurs. Adding Epsom salts or other magnesium sulfate compound to the water may help in dissolving the spines and reducing swelling. Vinegar, or urine, are not of help.
Taking ibuprofen or a similar NASAID may further reduce pain and swelling. Prescription topical steroid creams may give slight additional relief, but often do little.
Finally, apply a topical antibiotic ointment, but don't cover the wound. If infection develops, see a physician immediately.
BTW, there are anecdotal reports that crushing remaining spines by slapping them with a hard object has facilitated absorption. It appears that perhaps this has helped in some cases, but the fact remains that such a technique easily could drive spines deeper into tissue, potentially causing a problem requiring surgical intervention. It is not recommended.