I see you're using an ad blocker. I get it. I use one, too. But if you would consider enabling ads on this site, I'd appreciate it.
Alternately, consider joining my Patreon, which will give you the Keeper Edition, which has no ads and a few other fun features.
More details on the Mariner's Breastplate can be found at www.dndbeyond.com
When you view a magic item while the Detect Magic spell is active, you'll see certain auras based on the schools of magic at work in the item. For a Mariner's Breastplate, auras include:
Illusion
a basic brown with subtle hints of depth