  In this paper, we experimentally investigate the statistical distribution of
intensity fluctuations for underwater wireless optical channels under different
channel conditions, namely fresh and salty underwater channels with and without
air bubbles. To do so, we first measure the received optical signal with a
large number of samples. Based on the normalized acquired data the channel
coherence time and the fluctuations probability density function (PDF) are
obtained for different channel scenarios. Our experimental results show that
salt attenuates the received signal while air bubbles mainly introduce severe
intensity fluctuations. Moreover, we observe that log-normal distribution
precisely fits the acquired data PDF for scintillation index ($\sigma^2_I$)
values less than $0.1$, while Gamma-Gamma and K distributions aptly predict the
intensity fluctuations for $\sigma^2_I>1$. Since neither of these distributions
are capable of predicting the received irradiance for $0.1<\sigma^2_I<1$, we
propose a combination of an exponential and a log-normal distributions to
perfectly describe the acquired data PDF for such regimes of scintillation
index.
