As a long-distance competitive rower, I love the ViewPoint mirror by CycleAware. It's made for cyclists but works wonderfully for rowing. Best $14 that I've spent in along time. It's a mini-mirror on an adjustable pivot that adheres to the inside of your glasses. After a few rows it's very natural to glance and get a great forward view. I mounted the mirror on an older pair of glasses which I keep in the boathouse. I like it so much now that if I inadvertently forget to change glasses I almost go back to the boathouse. Fun to row up on a bridge fast and watch folks wonder if they should shout as you glide thru the piers without a glance over your shoulder.
Another benefit of ViewPoint: I train hard and don't like to wear a hat or sweat band because of the heat they trap so this little mirror is a cooler option. Its small size has another benefit in multi-rower boats. I stroke a double in a lot of open water races, including the CPR, and get bored not seeing where I am going so the mini-mirror gives me an inconspicuous way to keep an eye up front without glaring at my bow partner or tipping the boat to see ahead.