The dataset comprises the surface stability of sediments as determined by a Cohesive Strength Meter (CSM). Between 3 and 5 replicate measurements were taken from each of the 22 designated experiment quadrats at each of the Coastal Biodiversity and Ecosystem Service Sustainability (CBESS) sites. At each CBESS site, a salt marsh site and a mud flat site was examined and three locations were selected in Morecambe Bay, North West England and three locations in Essex, South East England. The Morecambe Bay samples were taken during the winter and summer of 2013. The Essex samples were taken during the winter, early spring and summer of 2013. This data were collected as part of Coastal Biodiversity and Ecosystem Service Sustainability (CBESS): NE/J015644/1. The project was funded with support from the Biodiversity and Ecosystem Service Sustainability (BESS) programme. BESS is a six-year programme (2011-2017) funded by the UK Natural Environment Research Council (NERC) and the Biotechnology and Biological Sciences Research Council (BBSRC) as part of the UK's Living with Environmental Change (LWEC) programme.