Form memory Chet went to the local garage and asked if he could "rent" all their 1 gallon screen wash containers for a couple of hours, and filled up the car with those to have enough ballast to "sink"
But floating / swimming is also an option
Back in the day (60's, maybe 50's ... probably in a Triumph Herald) , my Father told a story of getting marooned on a return trip from Scotland with all the border crossings impassable. A local offered to help, took him to an alternative crossing which was very steep sided and consequently the water was deep but narrow. "Follow me, you need to be doing 60 MPH" ... or something like that ... and they just aquaplaned across!