Not speaking from firsthand experience arriving by boat, but being a local who has travelled to the gulf islands...
Saltspring should have decent water.facilities, as should pender. Saturna probably not so much given how much of it is park land and marine
preservation area. Theyre all pretty close together and if you have a hard time go to the sidney marina, they are tapped into victorias water supply and its well equipped for even very large sailboats of significant draft
Avoid the government
docks as they rarely have water unless they provide permanent or transient moorage. Most of the ones next to a ferry
terminal are merely strips for loading and unloading only with no facilities at all. You might have time to walk up to the villiage that springs up by the ferry
terminal and back before someone is waiting (or yelling) that they need the space to tie up and load or unload. If you really want to walk around and have the space, stern tie to give yourself more time.