If you can build an earthbag water tank, then it should be possible to build an earthbag boat.
"This Instructable explains each step of construction for building an earthbag water tank using polypropylene tubing filled with crushed limestone. Moist, crushed limestone when compacted will turn into a stone like material, and get harder and harder over the years.
This water tank can be built in 2-3 days by four workers with minimal skill, although plastering skill is very helpful. You will need at least one helper to fill the tubes and help plaster.
1,056 gallon (4,000 liter) plastic water tanks retail for around $700 - $1,300, so at $200 this design costs far less than the main competing product. Unlike plastic, cement tanks won’t blow away in a hurricane, break down from UV rays or taint the water with plastic chemicals. Our water tank includes a domed ferrocement top that’s exceptionally strong. Tanks like this should last decades with only minimal maintenance."
https://www.instructables.com/id/Earthbag-Water-Tanks/
"This Instructable explains each step of construction for building an earthbag water tank using polypropylene tubing filled with crushed limestone. Moist, crushed limestone when compacted will turn into a stone like material, and get harder and harder over the years.
This water tank can be built in 2-3 days by four workers with minimal skill, although plastering skill is very helpful. You will need at least one helper to fill the tubes and help plaster.
1,056 gallon (4,000 liter) plastic water tanks retail for around $700 - $1,300, so at $200 this design costs far less than the main competing product. Unlike plastic, cement tanks won’t blow away in a hurricane, break down from UV rays or taint the water with plastic chemicals. Our water tank includes a domed ferrocement top that’s exceptionally strong. Tanks like this should last decades with only minimal maintenance."
https://www.instructables.com/id/Earthbag-Water-Tanks/