The coffer dam or dome as some call it is very, very heavy for more than one reason. The expectation is it will settle well into the gulf floor and hopefully make its own seal. Should it seal the recovery team can use the wells own pressure for the effort. Remember at this point we just want to stop or slow the flow of oil. Dealing with oil on the surface, or worse oil mixed with water and gas, is much more complicated.
Plan C is also interesting. The team would like to open on of the lower valves on the BOP and inject a combo of golf balls and shreaded tire with the hope of jamming up the partially closed BOP. Imagine throwing a bucket of golf balls and shredded tire in a flushing toilet. CLOG.