This makes no sense. A unit of 4 battleships with 3completely destroyed elements can be back to full strength in 30 days?
I'm pretty convinced at this point that there needs to be a major mod to overhaul the naval stuff. Off the top of my head, the best option would be each capital ship be a unit with one element. That way, when the unit loses its one element, it is destroyed.
Also, with the way the game handles stacks, this would work well. Both the british and the germans used 8 unit divisions for the battle lines. A leader + 8 one element units would make a stack. It would give more reasons to have more naval commanders, especially if stacking was turned one for naval units.