Not at all that simple. As already mentioned in the thread, they had limited fuel. Boiling 100s if not 1000s of gallons of water takes more fuel they already don't have.
SWAG: they probably weren't just using distilled water. There was likely some pH additive or other chemical treatment that water uses for a multitude of factors (scaling, corrosion, erosion, foaming, etc etc) that gets lost if you just vent water overboard - lost physically as volatile chemicals are vented or lost logically as new water must be treated.
Also, the boiling process itself is much more complicated than a pot and a fire. To achieve any reasonable amount of efficiency at all requires a multistage process with heat reclamation. If you plan on transiting salt water, the boiling process produces a thick, salty effluent that can be difficult to pump and is strongly corrosive. The weight, complexity, training, parts inventory, etc are all large costs. These are all hard sells for relatively short voyages.
Any ship installation is limited in size, weight, and power, so you carry machines only big enough to produce sufficient supply for normal operations plus a bit of spare capacity. Even if they had a distilling machine(s) on board, they would certainly not be able to supply as much water as they'd be able to boil off in the main boilers.
Also, the apparent ease of distillate production is a bit misleading. In fact, it is a bit of an involved subject, with multi-stage processing, and delicate balances of pressures and the occasional thorough cleaning. At least in the submarine world[1], that is...
Basically you want to heat up the water only as much as you need to, but later on recover as much as possible of the heat from both the distillate and the condensed discharge. This is helped greatly by maintaining proper pressures at different stages. Otherwise you'd be running exceptionally inefficient process, and at scale & over span of weeks things add up to real numbers.