I think I actually have to agree with BI on this one.
I will always look around and sum up the situation, but if I can plainly see an open slip close to shore and 5 or 6 sitting boats not doing anything about it I would assume that they are not interested and start towards it. Maybe I would ask, or at least make eye contact with, whoever is closest but I'm not going to flag down every other boat that was there before me and ask their permission. Now, if it just became available as I arrived then it's a different story. Then I would go through the normal process to make sure nobody else is interested first.