I had the problem for 4 years, remarked first time when the engine
had run only 100 hours.
I even had not only to fill the filter with water
(with the seacock closed) but also had to drain the seawaterpump by loosening the 4 little screws. I mounted a speedseal to be able to drain the watersystem faster and I'm happy to have that.
Always had a couple of full 2 liter waterbottles in the engine
room for filling the filter fast when it urged, ....changed waterhoses against steelreinforced, several new impellers, and lubricated the filterseal with silicone grease, nothing helped.
This year I changed the place of my vetus waterfilter from 30cm above waterline (mounted originally by HR to this place) first to 15cm above waterline like discribed in the vetus manual. As this did not help I lowered it further to about ~3 cm above waterline. When I look now from top in the open waterfilter I can see the seawater raising in the hose up to 2-3cm below the bottom of the waterfilter. This seems to be enough, since my engine D2-55 is able to suck the water
through even when there is no water in the filter.
But still I always listen and check if there is cooling
water spouting when ever I start the engine.
Meanwhile I have seen boats which had the waterfilter mounted below waterline and have to close the seacock for cleaning
I find it saver just above waterline.