Mere in English refers to a lake that is broad in relation to its depth, e.g. Martin Mere. A significant effect of its shallow depth is that for all or most of the time, it has no thermocline.
The word once included the sea or an arm of the sea, in its range of meaning but this marine usage is now obsolete (OED). It is a poetical or dialect word meaning a sheet of standing water, a lake or a pond (OED). The OED's fourth definition ("A marsh, a fen.") includes wetland such as fen amongst usages of the word which is reflected in the lexicographers' recording of it. In a quotation from the year 598, mere is contrasted against moss (bog) and field against fen. The OED quotation from 1609 does not say what a mere is, except that it looks black. In 1629 mere and marsh were becoming interchangeable but in 1876 mere was 'heard, at times, applied to ground permanently under water': in other words, a very shallow lake.
Where land similar to that of Martin Mere, gently undulating glacial till, becomes flooded and develops fen and bog, the remnants of the original mere remain until the whole is filled with peat. This can be delayed where the mere is fed by lime-rich water from chalk or limestone upland and a significant proportion of the outflow from the mere takes the form of evaporation. (Full article...)