i used an ultrasonic sensor for level monitoring in an aquarium project before..i don't know how well it would work in a boat sloshing around all the time though..
http://www.instructables.com/file/F6CDW7NI5KX6G91/
found this looking on nasa site
https://spinoff.nasa.gov/Spinoff2008/ch_1.html
http://www.instructables.com/file/F6CDW7NI5KX6G91/
found this looking on nasa site
https://spinoff.nasa.gov/Spinoff2008/ch_1.html