I don’t have experience with Marine simulation, but I ported a vehicle physics engine to unreal, and while it wasn’t trivial, it worked out nicely in the end.
I would recommend using the existing physics engine you have, running it on external threads, and just export the outputs of the sim to Unreal to visualize it. Unreal is a massive engine, and trying to integrate a new physics engine into it would be very painful. So instead I would keep them separate, and just use Unreal for input, audio and rendering etc, and pass data to and from your physics side.