Lawn fertilizers can wash through sandy soil and enter the lake relatively quickly, doing the same thing in the lake as they do to your lawn--make it green. Fertilizers are not needed, as there is plenty in the rain itself.
Naturalized shorelines are the best protection for lake water quality, and they have multiple benefits: native vegetation buffers can filter out nutrients in runoff so that they do not reach the lake, the vegetation can help reduce shoreline erosion, and these naturalized areas are much less attractive to geese.
Certain types of boats and boating use stir up the lake sediments and vegetation found in the lake bottom and also create waves which can erode shorelines. This reduces water clarity and increases the availability of nutrients which can fuel algae growth, and microbes which can cause rashes or infections.
Effective lake and watershed management is based on an understanding of the ecosystem, and this comes from long-term lake monitoring as well as detailed research projects and assessments. Human-induced changes to the system can then be evaluated and realistic management options considered.