Oil tankers, LNG carriers, and coal transporters are specialized to their cargoes. I doubt that there are alternative types of cargo for any of them. Certainly not in the same quantities. Transporting water, for instance, is not economic beyond a few thousand km.

For all the breathless talk (above) about how the cost of renewable energy and batteries are plummeting, it is still not clear how cargo ships will be powered sans fossil fuels, or what that will cost. Time and the market will tell, I suppose.