[ODE] How to simulate a sea environment?

[Emre EGE]

Hi;
How can i make a world that acts like a sea? That is the object will
act like underwater. Can i set the density of the world? (e.g. can i
set the density of water ). How can i set the forces over an object
while it is movinr?When an object moves underwater friction forces are
very important. But how can i simulate this? Please help me and if you
can send me a demo if you have, i will be very glad.
Thanks,

Emre