Underwater optical wireless communications (UOWC) performance is affected by turbulence. However, not much research has been carried out to estimate the probability density function (PDF) of the received optical power. In this paper, we investigate the effect of turbulence on the UOWC system using a new experimental setup with a variable link span in a water pool. Different turbulence levels are created by changing the temperature and the rate of an injected water flow in the pool water to obtain the PDF. Results show that lognormal distribution closely matches the measured PDF for a range of link spans. In UOWC systems, the link span is one of the main factors influencing fluctuations of the received optical power, and it has not been thoroughly investigated. In this work, the scintillation index and turbulence-induced power loss are obtained for a range of turbulence strengths and transmission link spans. Finally, we show that there is a good agreement between the experimental and simulated results.