caes hit the nail on the head (hope you have at least "one insurance" that will cover your injury), but perhaps some explanation is called for.

Among nouns, some refer to things that are countable, e.g., I own two goats, three pigs, and a [indefinite article indicating a single, non-specific example] partridge in a pear tree. Other nouns refer to this or that thing which cannot itself be counted; water, work, information, rice, music, rain, and - of course - insurance. In order to take an inventory of such things, other countable nouns must be added. For example, "a glass of water", "an item of information", "thirteen grains of rice", "two pieces [read compositions] of music", or "a drop of rain", and for our word of the day, "three insurance policies" [read contracts for insurance]

the use of "insurances" indicates a careless or uninformed author

As indicated by zipetaa's original question, both terms pertain to whether one party communicates to the other that he will not pursue a potential claim. Though not specifically distinct (their definitions in the well-regonized "Black's Law Dictionary" largely overlap), I believe that in practice they are used in different situations.

My understanding is that a waiver is used where there exists a legally recognizable basis for one person to assert a claim against another, but one has not yet exercised his power to make the claim and seeks to communicate to the other that one will not in the future exercise that power. On the other hand, a release is used where one has already exercised one's power to make the claim, but seeks to liberate the other from the claim already asserted.

Surely this doesn't cover the entire scope of the topic, but hopefully sheds some light on the distinction.