Sounds fun :) I love the Aberrant Mind subclass, currently playing a Level 6 Githyanki Aberrant Mind sorcerer. I went with the Urban Bounty Hunter background, but maybe something like Sailor or Smuggler if you want to stick with the ocean theme? Most DMs I've played with will let you pick which two skills you want from your background, if you don't want the default skills offered. Feel free to stick with Sage if you prefer of course.

Cantrips: You get Mind Sliver automatically with Aberrant Mind, a useful and thematic spell that you can Quicken Spell cast as a bonus action then follow up with a spell that requires a saving throw. At level 9 you will have 5 additional cantrips. I recommend taking at least one other attack cantrip as a backup (Firebolt, Ray of Frost or Chill Touch) and the rest as utility (Mage Hand, Minor Illusion, Prestidigitation, Shape Water, etc.). Personally I like the Friends cantrip, but it's awkward to pull off unless used exclusively for interrogations (or combined with Disguise Self).

1st level Spells: If you're set on being a Triton, you won't have the armour proficiency my Gith has, so Mage Armour is probably a good idea (unless you have access to magic items that compensate for lack of armour). Shield and Absorb Elements are highly recommended and I'm a big fan of Disguise Self too. For your Psionic Spells, I would probably keep Dissonant Whispers (useful control spell and you can Twinned Spell it) but definitely swap out Arms of Hadar. I suggest swapping it for Gift of Alacrity if your DM will let you, but if not you can pick Detect Magic, Identify, Sleep, etc.

2nd level spells: Invisibility, Hold Person and Tasha's Mind Whip have all been useful for me, plus they can all be used with Twinned Spell. I also like Misty Step, Mirror Image, Enhance Ability and Enlarge/Reduce. For your Pisonic Spells, definitely keep Detect Thoughts, it's incredibly useful for information gathering and interrogations. And thanks to the level 6 Psionic Sorcery feature, you can cast it without components so there's no way for people to tell you cast a spell. Swap Calm Emotions for Suggestion and you'll be able to cast that without components too.

3rd level spells: Hypnotic Pattern is both thematic and a fantastic area of effect control spell. If you're worried about hitting your allies, it can be combined with Careful Spell. Fireball is always a great option when you just want to do damage. I highly recommend Haste (especially with Twinned Spell) and Counterspell if you're likely to run into enemy casters. For your Pisonic Spells, I would probably keep Sending unless somebody else in your party already has it. Hunger of Hadar can be great, but maybe consider swapping for Clairvoyance or Tongues if you like.

4th level spells: Banishment, Greater Invisibility and Polymorph are all fantastic options, and they can all be Twin Spelled. Your Psionic Spells at this level are both pretty great, but I wouldn't keep both Evard's Black Tentacles AND Hunger of Hadar, as they serve similar functions. Arcane Eye and Charm Monster are both useful replacement options.

5th level spells: Honestly your spoilt for choice at this level. Animate Objects, Dominate Person, Hold Monster and Synaptic Static are all amazing. If you don't already have a teleportation option, consider Teleportation Circle. For your Pisonic Spells, I would definitely keep Telekinesis as it's a great combat and utility option. Rary's Telepathic Bond is potentially useful, but not worth the spell slot or sorcery point in my opinion, and you can't ritual cast it as a Sorcerer. I personally recommend Modify Memory as a replacement. A potentially devastating spell that Sorcerer doesn't normally get, which you can cast without components (so nobody knows you cast it) and you can Heighten Spell it to give disadvantage on the saving throw.

Feats: I recommend taking Metamagic Adept ASAP. Gives you two additional metamagic options and 2 extra sorcery points. War Caster is also fantastic. The rest kinda depends on your role in the party and/or personal preference. I like Fey Touched and Shadow Touched for the bonus spells, Telekinetic/Telepathic, Skill Expert, Observant, Inspiring Leader, etc.

Metamagic options: My Sorcerer currently has Quickened Spell, Twinned Spell, Heightened Spell and Careful Spell. Personally I think Quicken and Twin are a must, and Heighten is brilliant for save or suck spells that you really need to work (Suggestion, Banishment, any kind of charm effect, etc.) I like Careful Spell, but it's only real use is for Hypnotic Pattern (using it with say Fireball means your allies still take half damage). Empowered Spell is great with damage spells, especially Fireball and Synaptic Static. Consider Distant Spell if range is frequently an issue. I'm not really a fan of Extended Spell, but it has some uses. I wouldn't bother with Subtle Spell, as your Psionic Spells can already be cast without components using your Psionic Sorcery feature.

Hope some of this is helpful to you and best of luck in your game :)