Is this a YA book? Maybe Surfer Dude is really a teenager, a part-time lifeguard, but a surfer of legendary talent and daring. His father is the retired SEAL, and his father is the one recruited to lead the team... only his father is kidnapped by the evil team, and Surfer Dude holds the key to the first sunken city. The good guys try to get it from him, but he insists that he must come along, or he won't give them the information. He does this because he wants to rescue his father.
So we have a father and son. Need some YA subplots. Maybe Surfer Dude insists on coming along not because he loves his father (he does, but can't admit it), but because he feels a burning need to prove his worth and talent to his father. He has always been in his father's shadow, and now the tables are turned. Meanwhile, the evil team is led by none other than Surfer Dude's mother. Surfer Mom and Surfer Dad are divorced, so they can have a need-reconciliation or need-closure subplot, and Surfer Dude needs to reconcile with both of them over the divorce.
Why are they divorced? It can't be because Mom is evil, she needs a good motive. Probably has to do with why people want the artifact. Should be either crazy wealth or crazy power, and it's probably the latter. Maybe the first four artifacts assemble into a key (they won't recognize this at first, but will figure it out en route) that opens a secret vault in Atlantis, wherein lies the key to the great power of vril with which the Atlanteans built their world civilization and kept the ice floes at bay. Maybe Mom wants the power for herself, because she wants to save the world from anthropogenic global warming (or maybe, more generically, environmental disaster), and she calculates she can more easily do this from a position on the evil team; Dad, on the other hand, wants to do his duty by the good guys (a UN task force, maybe, or a US one, or GI Joe?). That gives them both "good" motivations. They quarrel, she leaves, he's disillusioned and quits the team, then later, both good and evil teams realize he holds the key, and come after him -- and the bad guys get to him first.
If it's about surfers, the book needs to feature cool surfing beaches. Maybe three aquatic locations per book, in a rough three act structure? For book one, say Malibu, Cornwall (could involve Tintagel Castle) and the Gulf of Cambay.
- Malibu Act: set up subplots, Dad is kidnapped, Dude can't quite save him, good guy team tries to get Dude's information, Dude holds them hostage to get himself on the team.
- Cornwall Act: the teams race and battle to get a piece of information (??); the bad guys apparently win, but due to Surfer Dude's ingenuity (??), the good guys get enough information to lead them to the climactic confrontation in the...
- Cambay Act: the teams battle, one of them gets the artifact (not sure it matters which, since the artifact could change hands back and forth over the series), but, crucially, Surfer Dude figures out where the next piece is hidden.
It's YA, need a first love plot. Surfer Dude should be in love with a bad guy (this will help him better understand his parents' dilemma, so the subplots will naturally intertwine as they resolve). He has fallen in love with a young bad guy agent, Surfer Chick, who has been hanging around the beach a lot, and who in fact turns out to be a scout / advance agent for the bad guys. (Perhaps he reveals the location of the second artifact piece to Surfer Chick in a romance subplot moment in Act III, in which he trusts her foolishly... but that trust will eventually win her over, down the line?)
There needs to be a bad guy team leader. He can be rich and funding the villains, and he wants the power of vril, too, for what he thinks is a noble end (a more extreme version of Mom's goal): he wants to "restore" earth to its "pristine" state, by reducing human civilization to rubble. Man has overturned the natural order with his tools and buildings, ending its own evolution by easing the pressure of survival-of-the-fittest. The way forward is to go back, destroy civilization, return humans to being cavemen in a wild and natural world.
Surfer Chick should be torn between Surfer Dude and Big Bad Guy romantically, so Big Bad Guy should be relatively young (which may mean his mistaken aim is due to naivete, and he can have a change of heart late in the series to help battle some common threat).
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See how I do it? Everything above after "Surfer Dude fights evil" is basically stream of consciousness. Since this is just a demonstration, I will stop it right here, but in a "real" project I would keep going and might produce three or four times this amount.
The next step will be to begin to structure this into plots and characters.