Hi Dougie,
I am pretty sure you are talking about "Doublers", which serve as stiffeners and/or connectors of two plates joining up and which can only be welded from ONE side. This is not strong enough - thus the riveting. Some of these doublers are also on the diagonal on the boats sides not only on the horizontal.
Back to your original question: Yes, the barnacles would also have attached themselves there. Rgds, RB