Saltmarsh peat deposits are widely used for the reconstruction of Holocene relative mean sea-level (MSL) change. Usually, the elevation of a peat sample is assumed to approximate the mean high water (MHW) level at sea at deposition time, but distortion of the tidal wave by local bathymetry can invalidate this assumption. This paper presents tidal data for monthly local MHW throughout the Great Marshes and Barnstable Harbor, Barnstable, Cape Cod, U.S.A. Significant temporal and spatial variations in MHW are observed. The temporal variations complicate the establishment of valid present local MHW levels to relate to fossilizing sea-level markers. The spatial differences in local MHW must be taken into account when off-shore palaeo-MSL is calculated from local palaeo-MHW. Maximum variations of up to 0.20 m were observed in monthly MHW in the tidal inlet of the study area, which correlate closely with variations in monthly MSL. Spatial differences are larger, however; monthly MHW levels near the upland border were up to 0.55 m lower than the simultaneous MHW level on the tidal flat, which, in turn, was generally 0.10 m higher than MHW in the inlet. Spatial MHW differences may explain some of the vertical scatter in the sea-level index points in the literature when samples originate from different locations. Even when samples from a single location are used for sea-level reconstruction, the presented local palaeo-MHW curve may deviate from the true off-shore palaeo-MHW curve due to a constant or time-dependent spatial MHW differences in MHW. Variable flooding characteristics of the saltmarsh surface also complicate efforts to refine sea-level reconstructions based on floral and faunal species-dependent relationships to inlet MSL. Moreover, the tidal measurements indicate a feedback mechanism between local MHW and saltmarsh surface elevation, resulting in a landward decrease of both in the most distal parts of the study area. These tidal measurements constitute a database to calibrate and verify an enhanced hydraulic simulation model for intertidal regions.