I'd go with a spray on type herbicide.
I used a granular Navigate product and it was very slow to work and then killed all the water shield in the pond on a temporary basis (I only placed it on one shoreline). The. areas it was placed was mostly wiped out at the root while rest of pond grew back somewhat.
It is also an expensive product.
I then tried Polaris and sprayed it in spots where I want to remove it - this is cheaper and easier to control.
I found liming my pond with pelletized lime made the bloom take off and the decreased visibility has put the water shield in check.
