you don't need to "get" them ... They're already in the water.
You want to find a plankton cloud ... Put out a light, at night, and they'll come.
Chlorophyll is in the plants & algae that are in the lake.
You don't need to scout for clear water/green algae ... Just the proper depth & cover.
Water run-off can be found where gullies run down the bank & empty into the lake.
Underwater cameras won't help you find plankton... Unless they have magnifying glass lenses. Algae will be on most obstacles that are below the surface of the water, but not out of range of the sunlight's penetration depth.
Class dismissed :p
... Cp