I have messaged several of you with a long write up of venues I found, but I just wanted to share 2 more I came across recently that might be very useful. They are up near Cecil County though, but it beats driving down through the tunnel or across the bay, or both depending on where you are.
1) Elk Manor Farm in North East has water and such, and they say they have a barn there but I can't find any pictures or anything, so I would just contact them and see. I also have no clue what the rental is but I think they allow outside catering which is a huge plus (As always I recommend KS Catering).
2) For $300, you get the entire weekend... Tea Barn at Fair Hill (I found this info out on the maryland dnr website, I think it's a state park, scroll to Fair Hill NRMA).
I found a youtube of a wedding there too, capacity is 125, someone on here said the dimensions were 35x50.
Alrighty, just trying to be extra helpful! It's super hard to find these rustic venues so if I come across more, I will keep them coming.