We took this trip at the weekend and it was great! We were out from 11am - 3.30pm and lunch was on board. We had a full commentary about the area (including the ships' graveyard) and had an hour at St Kilda to do the mangrove trail there (on the boardwalks).
It's perfect if you have family visiting who don't like to walk too far...!!
Spirit of the Port - Cruises Ship Graveyards Adelaide South Australia
It's not cheap but it's a great day out.