My daughter has 2 Cochlear CIs, so if you have any questions, happy to answer. If you take a look at DeafVillage.com and CIcircle.org , you should find a whole range of blogs listed representing all 3 of the CI makers.
My daughter swims in the pool and ponds with her processors on regularly, her preference. They are water resistant down to 30m [edit: sorry, that should be 3ft], for 30 minutes at a time. Never worried about kayaking, sprinklers, rain, snow, sitting up in the bath, etc., but we don't use them at the beach, in salt water, or in soapy environments (shower, under bath water). I think what differentiates AB's Neptune is that it may be OK to use in the ocean, soapy bath, or deeper still water, but I don't know for certain.
Honestly, I don't think you can go wrong with any of the 3, each has some benefits over the others, each has some relative drawbacks. And each is supported differently by your local CI clinic or audi staff, which is very important to look into. As you pointed out, your child's ear structure and hearing loss type may also determine the appropriate implant -- they are different shapes. That's probably the most critical consideration and one that only your doctor can help with.
