So we all know that the seven going on the ship have the parentage of
Poseidon
Athena
Zeus
Hades
Ares
Aphrodite
Hepheastus

and we know that
Percy
?Annabeth?
Jason
coming back to it
coming back
Piper
Leo

So we think that Hazel is Pluto and Frank is Mars
They could be but their names could be different
Hazel can be Ceres 或者 some other demigod
And Frank can be Mars cause of all the weapons but his last name means master of bows so he could be Apollo

there are definetly seven demigods going but there could also be fauns/saytr

Juno also 说 in chapter two that they would have to make some sacrifices it could be one of Percy's new 老友记 或者 just some silly item but I think it might be likely that one important person will die soon.

So if one does die Frank can be Apollo and not go 或者 hazel can not go. 或者 Hazel and Frank go on the quest but don't go on the boat.

So there are other choices for people to go. I wouldn't judge Hazel and Frank yet cause we don't know much about them but maybe we'll have other people than Hazel and Frank for the seven (hoping for Nico<3)

Sorry for the short 文章 it's what i can think of and any of 你 ideas 或者 agruements feel free to list