Hmm, depends on how handwavey vs. hardcore you want to be. Toting around water is a LOT more weight then L-Hyd, or even my preferred variant, solid hydrogen.
Classic doesn't seem to care, which makes water nice. MT doesn't care either unless you want agility, so long as you're using a maneuver drive; those are rated by spacecraft volume rather than spacecraft mass. Real life - real life would care a whole lot. Real-world physics would howl in anguish at some of the things we say we can do in the game.
We know the classic 1 EP power plant draws a ton of L-Hyd in 4 weeks, so about 414 milligrams per second, so the craft using that little plant would need to break about 3.7 grams of water per second and then either use or discharge the O2 produced. Since I think that would support about 360 people, I suspect most of the O2 is being vented to space. Of course, jumping is a different problem; we'd either need L-Hyd for that or home-rule some other way to power jump. Still might be worthwhile if we're saving some space on power plant fuel.
As for cracking water, well ... mostly the game gives us ducks without much attention to their feet: we see the ducks cruising through the water with only a general notion of how much effort they put into doing it. How big is the machinery that would be doing the cracking? I don't know, duck's feet. Are there dangers involved in cracking and then venting 2 and a half liters of O2 per second? I don't know, duck's feet. There may be dangers involved in the design of the maneuver drive too, or in the coolant system needed to support the fusion plant, or in the batteries that start up the fusion plant and that by canon can operate life support for a few days. There's something on that ship that goes kablooey if you make the wrong combination of rolls, but it's all duck's feet. We don't see it, we just see the effect of it.
That being the case, and Traveller maneuver drives holding physics basically in contempt, I can see definite value in taking 2/3 the room to store the fuel, so long as you're willing to give up the gas-giant-skimming bit. If we want the O2 vent system to have some interesting danger that might make for a bit of drama for the players, we can do that, but then we're landing ships that are putting out hundreds of millions of joules of heat per second without hearing much about dockside accidents, so maybe it's one of those duck's feet things we can bring up when we want a bit of drama and ignore at other times.
