The question is really vague, but if you throw a coin overboard the waterlevel in the lake will theoretically drop...
Lets say for making it more obvious the coin is replaced with a really heavy weight.
if the weight is in the boat it will displace a volume of water equal to the weight of it (since it floats)
^^this is a large volume
if you throw it overboard the weight will sink and will only displace its own volume (which is a smaller volume)
the waterlevel in the lake is determined by the watervolume + displaced watervolume by the boat or object....
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