thanks for pointing out that i mixed up the Ixx in the equation of center of pressure with the second moment of inertia of MBYou're getting confused here.
We're talking about the stability of this platform. The center of pressure, etc. has no bearing on this analysis.
The location of the metacenter M depends on the inertia of the waterplane cut by the hull of the platform. Using b = 0.3 would make no sense, since this dimension represents the depth of the platform's hull.
The Ixx used in the equation for yp represents something else and is not applicable here.
For the stability of this platform, BM = I / V, but I can be calculated about an axis which either runs parallel to the length of the platform or parallel to the breadth of the platform, which ever choice leads to the least value of I. V = volume of displacement and remains the same in any event.