High data rate underwater laser communications are highly constrained by laser propagation characteristics in the marine environment. We conducted communications experiments in freshwater and coastal seawater using an amplitude-modulated (i.e., pulsed) green laser beam detected by a remote optical receiver. We measured functional relationships between propagation distance, data rate, and error rate. Laser communications are degraded by absorption and scattering due to water, dissolved substances, suspended particulates, and marine biologics. Spatial beam spreading reduces the amount of optical signal that is collected by the detector. Pulse stretching temporally smears adjacent laser pulses, limiting maximum attainable data rate. Determining the capabilities of such a system requires characterization of the underwater communications channel. Relatively, little data is available regarding the relationship between optical water properties and the temporal behavior of laser pulses with pulsewidths in the nanosecond regime. We have conducted a series of experiments to measure both spatial and temporal properties of the propagating laser pulses. These through-water measurements have been made in the laboratory, in large fresh water tanks, in natural ponds, and in coastal seawater. We discuss spatial and temporal propagation characteristics of the underwater environment, and their relationships to the predicted performance supported by the communications channel.