In 16th century England bucket had an additional meaning (and in some parts it still has), that is, a beam or yoke used to hang or carry items. The wooden frame that was used to hang animals up by their feet for slaughter was called a bucket. Not unnaturally they were likely to struggle or to spasm after death and hence 'kick the bucket'. The link between buckets and death was made by at least 1785, when the phrase was defined in Grose’s Dictionary of the Vulgar Tongue: "To kick the bucket, to die." ( source )

The origin of this phrase is found in the book of Isaiah and precedes the phrase "a drop in the ocean." ( source ) It means what it did in Isaiah, a drop is a very small proportion of the whole.