I'm from Australia where we also boil water to make formula up. I had this question too, and had a big discussion about it on a different (non cruise) board.
What I discovered in all my discussions was our formula tins recommend boiling the water and mixing while still hot. This is, according to company websites, because they can't 100% guarantee that the formula is sterile. The US people on the board were horrified that this could even be a possibility and indicated that a company would probably never document this in the US as no one would use their formula.
Interestingly, the CDC in the US also recommends boiling water for formula, but I haven't been able to find a single person in the US who does. I'm sure they're out there, just not sure where.
Most people recommended to me to either use bottled water or just plain tap water. When I'm travelling in other parts of the US I will probably use bottled water if I don't have access to a kettle, given I won't know the quality of the water, but on the cruise I'll just use the tap water because of the whole reverse osmosis thing making it decent quality.
Oh - and I suspect a large reason that we boil and most people in the US do not is that in Australia and the UK we just about all have a kettle sitting on our kitchen bench. Not so in the US where it's less common to own one.