The seawater strainer placed above sealevel and on the highest point on the seawater system on our Yanmar 3gm30f is empty when the engine is not running.
Is this normal ?
NO, its not normal as the contents of the strainer can drain-back and thus leave the strainer body full of air. If that slug of air reaches to the raw water pump, the pump may have difficulty 'priming itself' all the way to the pump ... especially as the pump normally wears/ages and the 'sides' of the pump vanes no longer 'seal' the impeller to the pump body vs. air. Once the pump and strainer are fully filled with water by the action of the pump, the water in the pump will make the proper seal to the sides; if the pump 'air leaks' past the impeller sides you may be running 'dry' pump for a long interval.
A strainer 'should' be below the waterline.