Tottenham Marsh is part of the Lea Valley Park, a broad run of wetlands, grassland, towpaths, and open sky in north London. It is not manicured parkland; the appeal is the rougher feel of reedbeds, channels, and paths where the city loosens its grip for a while.
It works best as a walking or cycling stop along the Lea rather than a stand-alone destination. Bring sensible shoes after rain, and give yourself time to follow the water instead of treating it as a quick detour.