Well torque is r x F, where r is the red vector in your picture, and F is whatever force you're trying to apply. This cross product will generate a torque that can be used to modify angular velocity. I don't think you need to use approximation or point-masses since Stan Melax open sourced the code for calculating the inertia tensor of a model given the model's triangles.
You can treat your boat as a rigid body with a center of mass, inertia tensor, and mass. If you'd like to apply a force at a point on the boat you can do:
//--------------------------------------------------------------------------------------------------
void RigidBody::ApplyForceAtWorldPoint( v3 F, v3 point )
{
m_force += F;
m_torque += Cross( point - m_center, F );
}
To integrate the linear force and torque:
linearVel += dt * mass * linearForce
angularVel + dt * inverseInertiaTensor * torque
This means you need to calculate your boats inverse inertia tensor. Code for this is up on Stan Melax's website, he has a function that takes triangles and returns the inertia tensor. The tensor is best calculated in model space of the boat, and each can be transformed to world space by: inverseInertiaTensor = R * inertiaTensor * R^T, where R is the boat's orientation matrix.
Links:
Melax Volint
My implementation from Melax's page and Box2D
