Former construction material testing tech, here. Yes, the concrete absorbs virtually all the water added to the mix. In fact, the reason you keep it wet while it cures is so water doesn’t evaporate out and weaken it. That said, it forms a fairly small component of the total weight. Figure out how much concrete you need, by weight, and calculate the weight of water that went in it. If the water weight is more than you want to carry, mix there.
Numbers to use for calculating:
Concrete is usually about 145 lb/cubic foot
water is 62.4 lb/cubic foot or 8.338 lb/gal
There will be considerably less than 1 cubic foot of water in a cubic foot of concrete. Read the mix directions and estimate the percentages. Any other questions, I will check back later. I can even do the calculations for you if you give me relevant info. I used to actually make concrete mix designs.