And the problem of the weight to carry enough water for water needs and to convert it to Oxygen??
70 gallons of water at room temp weighs about 600 pounds. And how many gallons of water would a long distance space vehicle have to carry - for water and for oxygen?? So their is a weight issue.
Then at what temperature will it be kept. If a gallon of water is not kept warm in a space vehicle it could freeze. Then you have a different problem. A gallon of water turned to ice has more volume than it had as water - it needs more room. The practical implication of that adds to the size of the vehcile needed.
Then whether the weight of the water or the mass of the ship large enough to carry it, you have a question of the additional energy meeded to lift and propel a ship that can carry all that water in space. That requirement has to be considered also.
Splitting the atoms may actually turn out to be the easier thing to do than the practicality of employing it for spaceflight.
On the other hand, carried water has the flexibility to be used as radiation shielding (reducing net weight), can help with thermal regulation (reducing energy requirements), as well as be used for life-support both as water and by carrying oxygen in a denser from which requires much less in the form of pressure tanks (again, reducing weight).