The short answer is no. While splash-resistant with the MagSlider™ Lid, Rambler Tumblers are not 100% leakproof. If jostled in traffic or knocked over in the boat, some liquid will spill out due to pressure releasing inside the tumbler. For a drinkware option that is 100% leakproof, take a look at YETI Bottles and Jugs.
Please mid that all Yeti, delivery will be in JUNE 2021.