As to how long intact tins will keep food fresh I found this:

Wikipedia:

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The steamboat Bertrand, carrying cargo up the Missouri River to Virginia City, Montana Territory, sank on April 1, 1865, after hitting a snag in the river north of Omaha, Nebraska.

Half of its cargo was recovered 100 years later....

In 1974, samples of canned food from the wreck, including brandied peaches, oysters, plum tomatoes, honey, and mixed vegetable, were tested by the National Food Processors Association. Although their appearance, smell and vitamin content had deteriorated, there was no trace of microbial growth and the food was determined to be still safe to eat.

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