You are right that they represent protection and security; not only do they protech Pi from starvation at times, but he uses their shells as a "shield' from Richard Parker. Not really needed, but a comforting back-up plan if needed.
The turtles also represent Pi's forbearance, endurance, and triumph over difficulties. In chapter 66, Pi describes the huge feat it is to pull a turtle overboard. It takes almost a full page just to describe the endeavor. But Pi fashions a way to do it; it takes perserverance as it took a while. One turtle "hung from the side of the lifeboat for two days". After catching one, Pi would "fall back, exhausted but jubilant." Then, killing them required him to overcome his aversion to blood and violence. All of this asks Pi to forbear, endure, and triumph over difficulties.
Also, they represent a form of pleasure and happiness not often found out on the sea-Pi states that "Turtles...became my favorite dish." He goes on to describe the unparalelled joy-he "whiled away many a pleasant hour"- that eating a turtle became. He also used their shells for bowls, cutting boards, and shelter from the sun. So they provided him with great resources and pleasure too.