But to keep it from sloshing around like mad, you'd need a baffled storage facility. I imagine enough mass should (but doesn't in Trav) impact performance of your ship and its ability to be dodgy (inertia and all that). I can see cases where an incident, accident, or enemy action leave the cargo bay afloat with water a) trying to get into everything and b) washing its own mass plus anything it can pick up all around the bay (which might be bad for bay and contents).
If you spend the time (not being in a rush) to do the processing (or buy enough processor to do it quickly), then you could just deal with a small container of LH.
You can use some of your hold and get enough cracking gear to do it fast and still be cheaper than hauling all those dTons of useless oxygen.