Spare seat wheels
Salt water and sand don't mix with wheel bearings. We seem to be changing ours every 6 months or so.
Some touring groups will take a spare seat in an emergency bag just in case of a problem.
Whatever you do - its good to take a spare set unless you live next door to a rowing shop... which most of us don't.
Neaves Rowing are great for spares and advice. They also make a comfy foam seat that we are using in place of the carbon ones.