I love the NY to Norfolk/Baltimore shipping lane. Tons of tankers. Another good one is the NY to Halifax lane. These places have a TON of unescorted tankers and other assorted goodies poking along.
Basically anything off the New Jersey coastline is a good place to stay, in my experience. NY harbor is a very busy port, but there are a few DDs patrolling off the SW corner of Long Island that you have to watch out for. I haven't made it down to FLA or the Caribbean in any of my Drumbeat patrols as of yet.
You're right in worrying about the mud shelf off the coast of these areas though. But if it makes you feel any better, real U-boat captains did too. Adds to the realism.
I tend to stay far enough out that I can still patrol the shipping lane, but I can also submerge with no problems. Just mind your crash dives!