There is a resin model of a Russian armoured cutter (bronekater)
which was large motorboats armed with whatever they had handy, in this case tank turrets: https://www.hqhobbies.com/products/mig-productions-1-72-russian-bronekater-bk-1125-resin-ship-model-72098
I haven't got the faintest idea about what kind of boats they used to cross that river but maybe they used whatever floating hulls that were easily available, small barges, largish rowing boats, whatever was available in a river port/town.
Use any kind of suitably sized boat, think 5 to 10 meters or larger and divide by 72 to get 6.9 to 13.9 centimetres. The Bronekater is 31.5 cm so it is quite large. Don't know if it is still available though ...