Underwater Acoustic Sensor Networks (UWSN) is challenging research area due to limited bandwidth, low data rate, severe multipath, and high variability in the channel conditions. These complicated and non-linear channel characteristics render incorrect most simplifying assumptions used in simulations. We believe that, while researchers have proposed several novel protocols, their use of models and simulations as the only form of validation and intra-protocol comparison remains removed from reality. We argue that research experimentation is hindered by two fundamental constraints: high cost of underwater networking experiments, and lack of a single, easily-replicable platform for evaluation. We present here Underwater Platform to Promote Experimental Research (UPPER): a low-cost and flexible underwater platform designed to enable cost-effective and repeatable experimentation. We utilize commercial off-the-shelf (COTS) components to provide a HW/SW integrated solution that interfaces to two version of our custom hydrophones, from laptops that act as an Software-Defined Radio (SDR)-based physical layer, while allowing higher layer protocols to interact via a flexible API. With a total cost of $25 and $65 for each version of our underwater communication platform, we evaluate the platforms to demonstrate their data rates (50-600 bps) and range (5-10 m for v1, 30-50 m for v2), thus indicating a cost-range tradeoff. We believe our platform removes the barrier to validating simulation results in underwater environments while also allowing a fair comparison between protocols.