Dam..... hehe, I knew you were going to say that hehe..
Ok, my first thought is ALWAYS going to be about the bigger picture in terms of render-time.
My first initial thought would be to simply pick your favorite frame in your fluid sim, and convert your fluid to polygons, and shade it with "ice-like" shader system, or if you are adventurous, or have the render power, a sub-surface scattering shading network.
Although, you state that the ice shelf wont be the main detailed feature in your shot, so you can save render time, by just applying a generic ice shading network available in the Shader Library
Then you have your good looking "solid" ice shelf that you can layer on top of your current fluid moving ocean in your compositing app.
Then, assign a noisy gradient to "reveal" the frozen ice layer in your compositor to make it look as if that part of the water is "freezing.
Does that help?
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS
izzylong.com