On my engine Ford have designed an exceedingly tight fitting rubber plug cap. They are very difficult to remove. There are 3 or 4 separate little sealing fingers at the top which are a really tight fit into the bore. On their own they would make a formidable water seal. But first the water level would have to be deep enough to lap over the 10mm high upstand moulded into the rocker cover. That would be a big puddle of water sat on a hot cover and I’ve never seen a drop. But I do suspect that on mine the plug and the surrounding ali cavity below the cover is a condensation trap.
