There is no mast up storage on the north east side that I know of. I believe their is a primitive ramp on that side somewhere but I have never used it. Most people that sail on Rehoboth Bay launch from either the Delaware State Park "sailing beach" or Rehoboth Bay Sailing Association,
www.rbsa.org. Both spots are on the east side of the bay about one mile south of Dewey Beach. The state park has a daily entrance fee but there is no overnight storage. RBSA will sell you a daily or weekly use membership but not sure about overnight storage, you will have to contact them for details.
Do a search on this site, you will probably find more details.