There is so much confusion about this - are HCPs really bad at informing parents? I've breastfed first so haven't got the official message, but there seems to be a lot if different ways of doing it.
That aside, the key thing to know is that it's not the water that's potentially harmful for your baby, it's the formula powder. It's not sterile, and may contain quite nasty bacteria that do occasionally make babies sick. Hot water (>70℃) will kill the bacteria.
The WHO guidelines say it's OK to prepare a batch of bottles with hot water, cool down quickly and store at the back of the fridge to consume within 24 hours (that's what we did when I went back to work).
This, or preparing every bottle with hit water as you go along, are to my knowledge the only safe ways to use formula powder.
For going out, I'd use the pre-made bottles - carrying around a cooler bag would work of it's efficient enough, I suppose, but it is riskier. Formula is excellent breeding ground for bacteria, so i wouldn't risk it.