Lettuce production in southwestern Quebec represents an economically important crop for Canada. This vegetable is grown almost exclusively on organic soils in this region. Lettuce is highly sensitive to tip burn, a physiological disorder that can lead to significant yield losses. Tip burn losses have been linked to multiple causes such as inadequate water and calcium fluxes during transpiration, which in turn are linked to the water fluxes to and within the plant. The aim of this study was to investigate the dynamic link between water fluxes and the occurrence of tip burn in Romaine lettuce (Lactuca sativa L.) grown in muck soils. Water fluxes in the root zone were modeled with Hydrus 2D/3D using measured soil properties and weather conditions. The model results showed that soil hydraulic conductivity in the root zone was sufficient to meet plant water requirements down to a matric potential of −10 kPa. Field trials revealed that as soon as a cumulative hydric deficit of 3 mm relative to crop water requirements was reached, tip burn developed rapidly. Hence, the simulations supported the view that insufficient water fluxes are dynamically linked to tip burn in lettuce. A model integrating real-time field data and weather forecasts could therefore be used to anticipate water stress and avoid induced tip burn in Romaine lettuce.