In the paper, the possibilities of classic and laser fluorimetry in photosynthesizing organisms (PSO) diagnostics are analyzed. Some special features of fluorescence of PSO excited by laser pulses are discussed. The results of research presented in the paper refer to the water PSO -- phytoplankton. However, some of them are valid for vegetation. Methods of linear and non-linear fluorimetry make it possible to determine the following parameters. (a) Fluorescence parameter (Phi) o equals Nflo/NRS, where Nflo is the photons number of photosynthesizing organism fluorescence without saturation effect, NRS is the number of Raman scattering photons from water molecules. (b) Parameter of photosynthetic activity (eta) equals Fv/Fm, where Fv and Fm are the intensities of variable and maximum fluorescence response. This parameter can be determined by the pump-and-probe technique. (c) Excitation cross-section of fluorophor (sigma) . This parameter depends on energy transfer form auxiliary pigments and reflects the group and species composition of algae. The cross-section (sigma) and other photophysical parameters of PSO can be determined by non-linear saturation spectroscopy. In principle, these parameters can be used for the diagnostics of PSO, i.e. for the determination of chlorophyll-a concentration, of photosynthesis efficiency and of phytoplankton species composition. In the paper the examples of spatial distributions of parameters (Phi) o, Nflo and (eta) obtained by ship lidar and by submersible biophysical probe are represented. The results obtained in some sea expeditions using lidars and double-flash submersible fluorimeter for phytoplankton diagnostics, are briefly analyzed.