I'm going to be starting a new campaign next week. It is a pirate themed campaign so I figured it was a great chance to try out the new Fathomless warlock.

I'm not a min/maxer or optimizer in anyway so I could use some help with the build. I like the idea of mixing in some Storm Sorcerer? Does adding sorcerer to the fathomless make sense? It seems like being able to use metamagic would help with EB which seems to be the main way of doing damage for the Fathomless. 

I like the storm sorcerer for flavour. Controlling the waves and the sky seems like a really flavourful combination but can it work mechanically?

Well right off the tentacle is basically a noodley spiritual weapon so that is great.  The metamagic spell slots shenanigans with Warlock and Sorcerer is always fun. 

I don't know how you want to split the levels.  Part of me would rather err on the side of Sorcerer rather than Warlock.

Yeah mechanically sorcerer seems to bring more to the table with a dip into warlock. I was originally thinking of starting warlock and bringing in sorcerer but maybe I will switch it up and go sorcerer first with the patron coming in later.

Well certainly Con saves are great and come from the sorcerer.  

Sorcerer and Warlock are like two peas in a pod mechanically and the thematics of those two subclasses fit perfectly together. Seems like a really fun character concept. 

The most common split is warlock 2/ sorcerer X with warlock 3/ sorcerer X also being awesome. Warlock is super frontloaded, Eldritch Blast scales by character level and focusing on sorcerer means more spells per long rest and more sorcery points for metamagic. Fathomless seems like a natural fit for one of these progressions since the tentacle's uses per day scales by proficiency bonus instead of warlock levels. Sticking to 2 levels of warlock is the more popular of the two because putting off sorcerer spell progression by 3 levels is pretty harsh. But there are plenty of sweet benefits to choose from with that 3rd level and personally I like the idea of actually sealing a pact with your patron.

What level are you starting at?

Starting with Sorcerer 1 is excellent for Constitution save proficiency. Pick up Warlock levels after to establish the EB routine and then finish out with more Sorcerer. But there is something to be said for starting with Warlock and unlocking those abilities as quickly as possible. If you start at level 1 this approach gains more value, and if you're looking to take 3 levels of Warlock, then unlocking 2nd level spells as soon as possible is also nice. 

Warlock and sorcerer are a natural good mix, and the storm sorcerer would absolutely fit thematically with a fathomless pirate type of campaign. But the storm sorcerer doesn't really get anything interesting until level 6. I mean, before that all you get is the ability to fly just 10 feet after casting a spell. That's kinda weak. I mean, sure, by the time you get up to Warlock 14 Sorcerer 6 then you'll be one wicked cool pirate king (or queen, sorry) but that's a long way off. If you're planning on going pact of the blade then I might suggest dipping just 3 levels into Swashbuckler Rogue. That's a super-piratey flavor, giving you a boost to your initiative and letting you use Sneak Attack while dueling. And you'll eventually still get your 9th level Arcanum. So I'd aim for Fathomless up to level 4, then 3 levels of Swashbuckler, then the rest in Fathomless. 

But I've always been kinda partial to dragonborn, cuz they're wicked cool. So I'd also have to suggest a bronze dragonborn fathomless warlock with a dip into draconic sorcerer. But that's just me.