I think that you may be overthinking things. IMHO, it would have been easier to have Rocky go back to the docks and have Rubble help him load more tetrapods into his tugboat. Rocky would then go back to sea and continue to build up the barriers. In the meantime, Rubble would go back to the Lookout, pick up some more tetrapods and then bring them to the docks, ready for Rocky's next return. In that way, there is a continuous process flow until the barriers are high enough to stop the flooding. Afterwards, the flood waters can then be safely drained out by a huge pipe that Rocky and Rubble can both set in place with one end on the flooded beach and the other end out to sea. They would still need a giant pump to pump out the flood waters from the beach out to sea.