The current on the Spiegel Grove is usually pretty high. If you let go of the tag line on the boat at the surface, your chances of swimming back to the boat are not very likely on most days. If you are not used to this type of diving, perhaps a guide or a charter who is familiar with this dive would be in order. And if you know me, you know I hate charters.
When I go to the spiegelgrove, I either take my own boat or rent a boat. I motor out to one of several of the mooring lines on the surface. I tied my bow line to the mooring buoy and then I run a line from that bow line to the cleat on the back of the boat. That line runs a good 50' behind the boat. I jump off the boat fully geared, with mask and fins on, air turned on, ready to go with that line in my hand. DO NOT LET GO OF THAT LINE. I then follow that line hand over hand pulling myself slow but steady to the mooring line at the bow of the boat, then I follow that mooring line all the way down to the ship. DO NOT LET GO OF THAT LINE until you are touching the steel of the ship.
There is still usually quite a bit of current on the SG, but if you are good, you can shield yourself from it by positioning your body out of the flow. There is quite a bit to see from 80-100' and the sand is in about 137' of water. You definitely want a nitrox cert for this dive.
You can PM if you have any questions. I've been on the SG a bunch. I used to be a captain that frequented this sight almost daily.