Just remember, swimming in the algae won't hurt you, eating it does!
Lots of algae species are quite beneficial. 'Blue green' algae is one large group of algae that would have thousands of species; only some produce toxins. In fact, without algae, life on this planet might well cease to exist. So let's not be too hasty to hate it!
If you have an excessive growth of algae in a pond, you have one or more of these problems:
-too much nutrient input: Runoff from fields, etc. carrying fertilizer is sure to make your algae grow nicely.
-lack of higher plants: Have you mown the cattails that were sucking up those nutrients? Now they're all rotting in the pond creating fertilizer. Face it, plants of some sort will grow, so try to establish ones that are useful or beneficial. Livestock wandering around the shore of the pond will make higher plant growth virtually impossible, so use a water trough and keep the animals out of the water!
-lack of shade: in peak sunlight conditions, algae will grow and reproduce much faster than any other plants possibly can, so they'll use up any nutrients quickly. Having a few trees to shade the pond can be a big help- less light, plus if the water is a bit cooler that will help too.
Jeff Hathaway