freshwater lakes and resivoirs, including in-ocean
Storing large amounts of freshwater in natural and artificial basins is very common, and would be useful for our fairly arid islands. Less common, but highly useful, are in-sea water storage. Basically a huge plastic bag full of freshwater floating in the ocean. Presumably such bodies would be lined with something to prevent drainage and may collect rainwater automatically.
On-land water storage opens the door for hydroelectric power and pumped power storage. Conversely, undersea concrete domes are being developed to store pumped hydro.
Other bloat includes the concept of adding the option to use concrete slabs to build dams, or accounting for water toxicity when exposed to slag.