Dudley -- I think that I neglected to give you enough information. The holes in the tires are not simply to allow them to sink. When organisms, such as bacteria, build up on the inside of the tires, they release gases. Probably the most commonly released gas is just carbon dioxide, the same as humans breathe out. Other gases can also build up (examples might be hydrogen sulfide [that rotten egg gas smell] or methane). Anyway, as these gases build up, they will of course rise to the highest spot in the tire, and an unweighted or unstaked tire will actually float to the surface. So, that's the reason for the holes -- to let gases escape over time.
Dave
