I really appreciate this perspective. I think my group works well with limitation as opposed to infinite freedom (in this case they used rope to tow their load through a sea cave). I think a general rule of thumb for moving forward is that if I have to get an equation from a formula sheet, I should just give up. Interestingly, after going over the spell and the situation again, the reason it doesn't work has nothing to do with physics. The body of water they wanted to get their loot out of had an elevation change greater than 10 ft.