Here is a link to Mayflower Marina.
The Marina is just above the word "ferry".
For your hotel address in prospect, use Multimap and you will see the distance from your chosen hotel to the marina.
In general, I would say you need to drive, however, two points:
First is that the marina office staff will call taxis for you, no bother, the taxi ranks are v. close. and,
Secondly, there should be enough people around with their own cars to help us. When organising the South-West Cruise and the Scillies Cruise out of Mayflower, I don't think anyone has had to call a taxi yet !