I know this is an old thread, but I thought it would be helpful to update it with recent information.
The St. Peter is marked with a mooring buoy, usually May through October. The buoy is set up every year by one of the local divers here.
There are two ways to get onto the St. Peter. If you have a boat, Hughes Marina is directly south of the wreck. The ramp fee is ~$10 and there is ample parking for your truck/trailer. This is also the only place in the area to fill up with ethanol free gas. I sometimes take my 10' RHIB here on calm days, since the wreck is only ~1.5 miles off shore. If you do not have a boat, starting this past summer there is now a shop running occasional charters to the wreck. The name of the shop is
RU4SCUBA. They ran a few charters out there this past summer and probably will be in the coming years. They have a large pontoon boat that is docked at the marina there. The shop is outstanding, and is more than willing to arrange a trip out to the St. Peter, even if its just you who wants to go. Cost is extremely reasonable as well.
Expect a wreck that is very cold and usually dark. This summer the warmest I saw was around the low 40's. Visibility was close to 100 feet in May, with the worst observed in August at around 10-20 feet. Although not usually mandatory, I would suggest bringing a light so that you can poke around a little easier. Here is a link to a video from a recreational dive I did on her this past summer:
St. Peter Wreck Video (YouTube)
The deck of the wreck sits at 100' and the bottom is around 125'. Due to the cold, I consider diving doubles or having a bailout to be mandatory. For those who want to deco dive the wreck, there is a great up-line which is very easy to find (it's secured to one of the masts).
There are also two other dive sites in the area, both with mooring buoys. As you are heading north out of the marina, you'll first encounter a blue and white buoy in ~25' of water. This is a small "playground" type area for open water classes. Traveling further north you will next encounter a tall yellow buoy with a diver down flag attached. This marks the wreck of a small sailboat (about 25' long). This wreck sits in ~45' of water and makes for a great second site after the St. Peter. If you continue north, you'll find the blue buoy marking the St. Peter.
I hope this information is helpful, feel free to contact me if you have any questions!