They are using water to put out the fire in the water.
This, although with a large enough volume of gas (which there obviously was), that water just turns into fuel with all the combustible gas dissolved into it. So more water could theoretically dilute the mixture enough to keep it from spreading or extinguish it.
Still, these offshore operations seem really sketchy, based on what I've read over the years.
Like the lesser-known, but still insanely damaging Taylor oil spill, which also happened in the Gulf of Mexico, and is still going. It's the longest-lasting oil spill in US history. The industry is just a complete joke, as far as regulation and oversight.
At least this one here does indeed seem to be gas, given how clean the water appears, but who really knows. Underwater blowouts can drift for miles before surfacing, and gas is more buoyant than oil. It's hard to take anything said by officials at face value, given the history of the industry.