My reply may be too late (Have you already decided? What did you choose?), but having used an E-PL5 with the 60mm macro lens for many years, I would say that for general underwater macro the TG-5 is a better shooter, despite the smaller sensor. A smaller sensor brings a crop factor that effectively increases macro capability, and the TG-5 "Microscope" mode is outstanding for a compact camera. Additionally, the variable zoom of the TG-5 makes it more versatile underwater. With the E-PL5, invariably you'll fit the 60mm macro lens to go shooting nudibranchs, and then run into a big shark or ray and about all you can photograph is its eyeball. Exaggerating, but you get the picture. The TG-5 at least lets you vary between wide-angle and macro on the one dive, though its wide angle image quality is not so great. Plus it's smaller and more portable for travel.
If all you're going to do with the photos is put them up on Facebook, then I say stick with the TG-5. Use the money that you might have spent on the E-PL9 and buy yourself a diopter/wet lens and make the TG-5 even more flexible underwater.
On the other hand, if you want to maybe make some prints from your photos, or you want a good wide-angle experience (with an additional lens), or you want proper manual control over your photos, then the E-PL9 and the AOI housing is a fantastic option for the money! There are some great high-end features in the camera, and interchangeable lenses does bring a lot of scope for different types of photography. (It's just a pity you can't change lenses underwater - wet lenses aside). Do note that the AOI housing uses inbuilt LEDs to trigger strobes, not your camera flash. The battery that powers the LEDs and the moisture detection circuit is an inbuilt rechargeable LiON battery, which may not be user-replaceable. Those may or may not be drawbacks to you.
