In addition to commenting on misguided logic, I'm also the OP

I would call your attention to two phones and a two way radio as examples of how to design a removable battery into a waterproof device.
First, look at the Samsung S5. It is waterproof (IP67) and gets decent battery life with its user replaceable 2800 mAH battery.... it does it with a gasketed rear cover. It is very thin and in fact if it had a slightly smaller overall outline and an unlockable bootloader I might be using one.
Second, consider the LG G5. It is not currently waterproof but the replaceable battery slides in and out of one end of an easily sealable enclosure and it would be very easy to gasket the removable end cap and thus render the battery compartment waterproof.
And lastly, I own a waterproof ham radio walkie talkie. It achieves its waterproof status by making the battery the whole back and placing a simple gasket between the battery face and the radio body.
Any of those approaches would produce a waterproof phone with a removable battery with currently available tech and minimal (if any) reduction in battery capacity. Who knows what might be possible with a little actual thought?
Paul