Indigenous Services Canada partners with first nations, and first nations implement the response. I won't speak to the governance practices that happen in first nations communities or in self-governing first nations.
There are interim supply options when a long-term water advisory is in place. For example, one that's often pointed to is bottled water. That is an option in order to provide water in lieu of having it from the tap, but there are other options as well. Many communities have reverse-osmosis systems in order to provide water in lieu of water from taps. There can be interim measures to replace or repair existing infrastructure while other infrastructure is replaced.
The binary choice of having a boil water advisory or a long-term drinking water advisory and not having water from the tap is sometimes not the case. There can be interim supply options. There can be bottled water provided to the community, based on the request of the chief and council, while long-term solutions are put in place. The plan considers not only the long-term replacement of the particular asset or infrastructure in question but also what the interim supply will be in order to provide water now while also looking at the long-term picture.