Dealing with a wet backyard. Ocean side property and near a River

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Has anyone dealt with an area in their yard that is always wet? The Single Family Rental house is along the shore. Not far is the Neponset River so based on that geography the developments in that area are on marsh land I believe from way back when and prone to be wet. The yard slopes slightly downhilll and abuts a wetland sort of area with tall reeds overgrowing on the other side of the fence. I think it's a real eyesore and would love to rehab the landscaping to make it more appealing for a future sale but don't want to spend a ton of money. I'm thinking of installing a rain garden in that corner of the yard as it is basically unusable for anything else. Thinking rock garden, water tolerant flowers and shrubs. Landscapers with sustainability in mind  with this experience would be helpful. 

Best,

Justin