OK, job done, but in case its any use to others...
Can be done with boat in water
, no problem. Some water will leak out, but not enough to worry about and it is easy to get the grease in against the water flow.
Couldn't find Mac Donalds here, but discovered that a dinking staw from just about any other outlet works just as well, as long as its flexible and can be squeezed flat.
Fill it with waterproof grease (I used the Volvo Penta
Squeeze seal to open it slightly at top (this is when some water comes in).
Insert flattenned tip of straw just about 5mm and squeeze all the grease out of straw into seal.
Extract staw and repeat until about 1cc grease has been injected.