@Eviltwin
Normally, Mahiro uses “Ore” when alone or with Mihari, but he uses “Watashi” around Momiji and the others to look more like a normal girl. It’s a notable detail in context.
@Extralie‘s google results are pretty much correct. A guy can use “Watashi” in a business context like during work, but it’s a bit weirder to use that in a casual context.
EDIT: Oh snap, I just realized that the line itself is just a reference to a movie called “I Want to be a Shellfish”, so the use of “Watashi” here is more likely just a joke rather than an actual slip or anything. That’s a big mistake on my part, sorry.