From Jerebear's Carry Pet Experience Reference:viewtopic.php?f=10&t=8829
For Pandaria tamers:
experience = 12.5*[ (your_pet_level + 9)*(30 - your_pet_level) ]
I just did a Shipwrecked Captive battle with a level 21. No hat, no buffs.
I got 2025 XP
That works out at
experience = 7.5*[ (your_pet_level + 9)*(30 - your_pet_level) ]
7.5 is the multiplier used for pre-Pandaria tamers.
I see comments saying that he used to give better XP. I assume his factor was nerfed from 12.5 to 7.5.
I actually regard this as good
news for people who are levelling pets. With Squirt, Blizzard restricted the cheap levelling to one day every few weeks, and that day was a nightmare. I might have expected Blizzard to restrict Shipwrecked Captive to once per week, or perhaps once per day, or once per week per character. Instead they seem to have nerfed him to give 60% of his original experience, but he can be battled at any time, as much as you like. I'd call that a good deal.