I haven't played Pathfinder in years and the d20pfsrd is a little overwhelming with all the content that has come out.
My son and I will be playing in a water based campaign with 4 other players, so I'm looking for two different class/build suggestions.
The other characters will be a gunslinger, bard, druid, and an undetermined. We are starting at 3rd level.
For myself I am looking for a tanky melee build that doesn't rely on heavy armor. I don't want to drown!
I'm open to anything for my son, but he's 8 so the less complicated the better.
I'd consider a bloodrager and eldritch scion magus both with the aquatic bloodline. At 4th both would have waterbreathing and a 30' swim speed. Both can be moderately 'tanky.' While the magus is more complicated in theory, in practice it is often a few go to tactics via spell combat/spell strike and isn't difficult to figure out and can give a nice 'nova' effect that your son might like. The bloodrager would be more steady and dependable damage, and that might suit you well.
This would also let the two of you work together a be a team.
You have a ranged specialist, support, and a 9th lvl divine caster.
You're missing a 9th lvl arcane spellcaster and a melee presence.
So maybe Sorc, Wizard, or Arcanist might be in order for your son. I think your son might enjoy Sorc best because it's much less complicated than the Wiz or Arcanist.
As for you, I agree with Dave Justus, Bloodrager or Barbarian.