I see no one has replied. I am contemplating the same thing. It's really only an issue if you frequently go through the surf zone. To this point, I've always carried a large sponge and simply sponged out any water that I take going through the surf. Presently, I'm working on the electronics for my PA. I have the 12v in the rear hatch in a modified "bucket" that fits that hatch. I'm waiting on a charger controller at which time I will be rigging a solar panel to keep the battery charged. I am presently only using it for a bottom finder. I am still on the fence about installing an automatic bilge pump. Most that I have seen do this send their discharge to the rear deck, which drains via the scupper holes. Frankly, I think it's a bit overkill, as I've never taken that much water in, even when I had cracks on the deck. That being said, most of the new models of PA's come with enough flotation installed that even if they were to become completely swamped in the inner hull, they would still remain bouyant. At least, that's my understanding, though I've never approached that level. Having said that, I have had a gallon or two in the hull and the performance was diminished considerably. Perhaps you might consider a manual bilge pump, which is available from most any kayak source. They are typically used to bilge sit in kayaks, and are supposed to be quite effective. Just a few ideas. I wish I had some pictures posted and will try to do that once I get the rest of the parts in for my electronics set up. In the meantime, here is a facebook page of a fellow who has recently mounted his battery in the manner I describe. Might give you some ideas.
http://www.facebook.com/EverythingKayak?fref=ts