Genome size was assessed by flow cytometry in 55 populations of 53 taxa with different kinds of relationship with freshwater environments (25 populations of aquatic plants, 24 of wetland plants and six occurring both in aquatic and wetland habitats). Nuclear DNA content is provided for the first time for one family, five genera, 28 species, four subspecies and one variety. Around three quarters of the results obtained belong to the lowest levels of nuclear DNA content (very small and small values), which agrees with the large genome constraint hypothesis. Nevertheless, some remarkably high values were also found. Further studies would still be required to increase the set of water-dependent plants with known nuclear DNA amount in view of clearly establishing any relationship between genome size and aquatic habitat. A synthesis of the current knowledge on genome size in water-dependent angiosperms shows a distribution of C-values biased towards very small genomes, a tendency especially strong in wetland plants, where species of 2C < 1 pg are four times more frequent than in angiosperms as a whole.