In this work we present optical pH sensors especially designed for pH measurements in marine environment. Embedded in an uncharged, highly proton-permeable hydrogel matrix, the two novel lipophilic carboxyfluorescein derivatives 2',7'-dihexyl-5(6)-N-octadecyl-carboxamidofluorescein (DHFA) and 2',7'-dihexyl-5(6)-N-octadecyl-carboxamidofluorescein ethyl ester (DHFAE) have apparent dissociation constants of ca. 8.4. The pH transition range of the sensors perfectly matches the pH range occurring in seawater and marine sediment (ca. pH 7.2-9.2). The cross-sensitivity towards ionic strength (IS) was found to be low for DHFA-containing membranes and was even negligible when using DHFAE as indicator. The quantum yield (QY) of DHFA (0.94(basic)) is similar to that of fluorescein (0.97(basic)). QYs of 0.62(basic) and 0.22(acidic) were found for DHFAE. The optical properties of the indicators enable referenced measuring schemes. Lactonisation of the DHFAE chromophore is prevented by esterification of the carboxyl group in 2 position. Thus, internally referenced dual wavelength measurements are possible since the emission maxima of the basic and acidic form of DHFAE differ by 30 nm. Dual lifetime referenced (DLR) measurements were made with pH sensors incorporating ruthenium-(II)-tris-4,7-diphenyl-1,10-phenanthroline (Ru(dpp)(3))-containing reference particles in addition to the indicator. This type of sensor can be applied for pH imaging or in phase-modulation measurements of pH.