Results are presented from infrared transmission measurements over long ranges at low elevation in coastal waters. This work was part of the EOPACE programme, conducted at two locations along the US West Coast. In this programme data were collected for validation of atmospheric models, used to predict IR sensor performances Details are given on the transmissometer set-up and measurement methodology. The data concern midwave (3-5 um) and longwave (8-12 um) measurements, taken over the Monterey Bay (22 km) and San Diego Bay (15 km) in various periods of the year (March, April, August and November). The results are compared with model predictions, including molecular extinction, aerosol scattering and refraction effects. The first two effects have been determined carefully and could be quantified to a certain extent with the help of simultaneously collected meteorological data. The refraction effects are of a more complicated nature due to the complex temperature profile structure over water, which is difficult to measure.