If they have access to any of the water, they have access to all of the water and so do their guests. Any owner could be held liable for any mishap. Also, there is trash, noise, etc. What if the other owner wants to let his relatives' children use a jet ski, as an example? It's a substantial pond or small lake.
Covenants and agreements are really hard to enforce, and would require a court order.
What a can of worms joint ownership can be and often is.
