I've done Annapolis to New England via the C&D Canal and Delaware Bay three times on different boats, all trips in mid June. From Cape May we went offshore to Block Island, Narraganset Bay, or Buzzards Bay.
Each time, conditions were different.
The first trip, we went through the canal at night, nonstop from Annapolis, and anchored in the Delaware River just outside until daybreak. Conditions on Delaware Bay were benign, and we went right out of the Bay into the Atlantic without bothering to stop at Cape May, turned left and headed for Block Island. Pretty good sailing.
The second trip, we anchored overnight somewhere before the Canal, then in the Maurice River on our way down Delaware Bay, and then from there went in the "back door" to Cape May. This was mostly "motor sailing."
The third trip (this past summer), we started down the Delaware River (after leaving Chesapeake City around 07:00) with the tide against us. The wind was blowing out of the southeast at 25 knots, right up the Delaware Bay. Around mid-day the tide turned in our favor but with the wind against the tide, the waves started to develop. We saw some of the famous Delaware Bay "big square waves," perhaps approaching 8 feet (mostly more like 5') but it was manageable (just hard slow sailing). We anchored off the beach on the west side of the Cape May pennisula around midnight, after beating all day, and then went through the Cape May canal the next morning. The winds were still strong out of the southeast, and there was a good swell running, so waves in the Cape May inlet were pretty interesting, but we punched through all right. You can read more of our logs from that trip here: http://sailing.thorpeallen.net/Greyhawk/Delivery/