Lava is hot. Super hot. Hell is colder than lava.
Water could conceiveably cool it down, and being an island, there is an endless supply. Some quick googling brought up an instance in Iceland where they did exactly that.
But here's the thing: while the water and steam could help dissipate heat at the leading edge of the flow, you might end up diverting it into an unexpected direction.
In Iceland, they were going to lose an important shipping port. That makes for a serious economic impact, well worth the effort.
With Kiluea, they'll watch some homes burn.
Yeah brah! Evict those haoles with fire!!