should I top up with water
 

I just put two batches to secondary. My carboys arent full to the bottom of the neck . Should I top up with water, I do this with wine but I dont want to make a weak beer. Its a 5 gal carboy .

if you are to the shoulder at least i wouldn't worry about it.
there should be a slight amount of c02 produced in the secondary to displace the small amount of air in there.
for better or worse i swirl the secondary a bit after the transfer to release a bit of the c02 into the headspace.
i would not top up with water personally