Read about Density in this short article from Britannica Online:
http://school.eb.com/levels/middle/article/599545
Buoyancy and how ships float:
https://wonderopolis.org/wonder/how-do-boats-float
How does a heavy boat float:
http://www.boatsafe.com/kids/021598kidsques.htm
Why boats made of steel can float:
http://science.howstuffworks.com/science-vs-myth/everyday-myths/question254.htm
Video: Why do ships float - 1:39
https://www.youtube.com/watch?v=-ykZsC5M2lg
Video: Displacement, shape and air - 3:32
https://www.youtube.com/watch?v=FAAgM_fRhGU
Video: How do ships float? - 1:00
https://www.youtube.com/watch?v=xniW3_afO-0
Video: Why don't big boats sink - 2:49min
https://www.youtube.com/watch?v=Yyvyj41njBs