Sunrise can be tricky cause it's hard to tell how bright it will get once the sun comes up, so using auto iris isn't a bad idea. I've shot a few of these. I remember the first time i did it I kept having to adjust the iris manually and it came out pretty bad. With sunset you setup while the sun is still out so you know where to set it. Though if on the west coast and want to see more of the ocean I'd also use auto iris. Otherwise it'll just go really dark after the sun sets.