This should be your guide. However, do not be surprised if most of the older cottages have smaller lots. The smaller the lot, the more careful you must be. This question has 2 parts to it. First, the question of contamination, that is the presence of germs in the water.
Swimming is forbidden when the water is contaminated.
The safest way is to bring a sample of water to a laboratory for analysis. The owner may have some recent results to show you or he might tell you about his personal experiences. This is the second part of the question. Everything that decomposes in the lake water, whether it is human waste or organic matter, enriches the water and promotes excessive growth of aquatic plants. You must not forget that aquatic plants are an essential component of a healthy lake and you should not be alarmed by their mere presence.