Concrete Block Raised Pond - Will it be strong enough?
Not sure what it might be called where you are, but there's a &quot;reinforced stucco&quot; (might be called reinforced render in where you are) that can be used to drystack blocks and then bond them together as a unit - parging a layer of that around the inside would provide some structural benefits. If you can't find the product as a product, you might be able to find loose reinforcing fibers for cementitious products and add them to a render/stucco coat for the same effect. &quot;Surface bonding cement&quot; seems to be another name for it. Third possibility - the render coat you have might already be reinforced?
As to whether or you need to do anything, partly depends on frost movement or lack thereof as a concern - you are correct that it's not very deep, so the pressure is quite low, but unreinforced masonry has very little tensile strength. If the water was not drained and it froze, you can just about guarantee that it will crack, unless you shape it to have tapered sides. Even emptied, movement from frost in the ground will usually crack unreinforced walls. If frost is unheard of in your location, that might be a reason not to bother doing anything. Since you are using a liner, tapering the sides should be as easy as building up sand against the wall before placing the liner.