There is a technique whose name I cant remember for older clinker boats which involves pinning a 1/4round wood batten same thickness as the exposed plank edge, on sealant down the lands of each plank, effectively sealing it. Tried it once myself on a leaky old clinker rowing boat - surprisingly effective, but spoiled the appearance a bit - much less than sheathing though!

Common practice for elderly clinker working boats, and clinker ex ships lifeboats coming towards the end of their days until GRP took over.