The most obvious turtles are the Lego turtle that Bill is holding when he sees Ghost Georgie, as well as turtle wallpaper in Bill’s house. The other turtle reference is something that doesn’t feel out of place until the scene ends: When Beverly has joined the Losers for a swim in the quarry, they notice a turtle and all dive beneath the water, portending what feels like a jump scare waiting to happen. Then... the scene ends, and we pick up after the kids are done swimming.
Muschietti was acutely aware of the effect of the line and the edit. “The screen goes completely green, and I think people sort of brace because they think something scary is going to happen, but then we cut to the next scene. It's a bit of an Easter egg for people who've read the book. For people who don't know the story at all, The Turtle goes completely unnoticed, I think. But I didn't want to leave The Turtle completely out, because I think it's a hint for further exploration in the second part."