I think the USB & speaker are the weak links for water ingress. Also, a removable battery would (probably?) significantly weaken the phone. So, if you dropped it, it'd be more likely to sustain real damage.

I don't see them as very big weak points. USB doesn't have enough voltage to do jack in water even if you don't detect the water and turn it off. And the speaker can be made entirely out of waterproof materials, there are literally waterproof floating pool speakers you can buy for dirt cheap. The weak link is the main oring/glue as always.