there is a shell with a pearl in it and a quote on the bottom that says, to find the pearls you need to dive

Stella Marina

They are given a magical pearl from a Myriad (a sea messenger) to save themselves from Hades. They then also discover that Ares has the bolt and that Percy didn't steal the helm of darkness. Percy challenges Ares to a duel and is victorious. Hades then returns Percy' mother safely.