For an area in a world I am creating, 300 years prior to the present day, the northlands were destroyed by major volcanic eruptions. Would it be possible for there to be water accessible to a party journeying through these 'Ashlands' or would there be too much volcanic contamination?
Yeah! I think fresh water sources like rivers and big lakes will provide quite potable water.
300 years is a very long time. After 300 cycles of rain, water in surroundings will no longer be acidic due to all those oxides of sulfur and nitrogen. The SPM will settle down. That means, no longer turbid water. In fact, life would be flourishing because of rich nutrients which were ejected in the eruption.
In a nutshell, 300 years is quite long time for any contamination to persist in ecosystem, answer could be different for less than 50 years.
300 years is more than enough. In 1985 Nevado del Ruiz volcano erupted. The near city of Manizales wasn't even affected and the main source of production in the city is coffee. Deep containers of water can be blocked by the eruption rather than contaminated. And remember that acidulous waters are considered spa waters.