Why would someone specialise in repelling blast? No idea but you're a sorlock anyway so why not do something funny like push a minotaur 300 feet away.
Sea Sorcery Sorcerer 5 - Warlock 3 - Sorcerer 12
If you want repelling blast to come online as early as possible, instead: Sea Sorcery Sorcerer 1 - Warlock 3 - Sorcerer 16 - but it slows down your access to spells but then you're just a cantrip spamming machine anyway.
Sea Sorcerer 1 gets you:
Curse of the Sea, which does some cold and lightning stuff, but really what you want is the forced movement portion.
Curse of the Sea
When you choose this origin at 1st level, you learn the secret of infusing your spells with a watery curse.
When you hit a creature with a cantrip’s attack or when a creature fails a saving throw against your cantrip, you can curse the target until the end of your next turn or until you curse a different creature with this feature.
Once per turn when you cast a spell, you can trigger the curse if that spell deals cold or lightning damage to the cursed target or forces it to move. Doing so subjects the target to the appropriate additional effect below, and then the curse ends if the spell isn’t a cantrip (you choose the effect to use if more than one effect applies):
Forced Movement. If the target is moved by your spell, increase the distance it is moved by 15 feet.
So, if you cast a cantrip that moves the target the distance is increased by 15 feet, this curse lasts on the target until the end of your next turn and it does not cost an action or bonus action to trigger.
Prerequisite: Eldritch Blast cantrip
When you hit a creature with Eldritch Blast, you can push the creature up to 10 feet away from you in a straight line.
So each hit of eldritch blast pushes back a creature by 25 ft. At level 5, if you hit a creature with an eldritch blast twice they'll be pushed back 50 feet, which means most melee types won't be able to both close the distance with you and attack in one turn. Level 11 you're pushing creatures 75 feet away, and at level 17 you're pushing creatures 100 feet away.
I know I know, I said 300 feet. So how? Well, We're still a sorcerer so we could quicken eldritch blast, that gets us 100 feet at level 5, or 200 feet at level 17, but that's not 250... so how about we take some feats we were probably taking anyway and one extra.
Warcaster, yeah advantage on con to maintain that twinned haste on the fighter and barbarian. Always solid.
Spell sniper, cast out to 240 feet for eldritch blast, we're pushing people out of this encounter and into their own one. Alternative, Eldritch Spear for 300 ft range but doesn't help with cover... or do both for 600 foot range, perhaps using the Eldritch Adept feat so we can keep Agonizing blast. (maximum range here would be 1200 feet with distant spell metamagic but that's overkill).
Polearm master, Someone runs into our reach and thanks to warcaster we can tell them to leave this place and never return by casting eldritch blast as an attack of opportunity.
If this hits, they go back 25 ft, and depending on our level we can do this again. If we're level 17 we push them 100 feet. Then on our turn (the curse of the sea lasts until the end of our next turn and starts again when we hit with a cantrip anyway) we can push them another 100 feet, plus we can quicken another 100 feet. Assuming every bolt hits our attacker they are now 300 feet away or if their speed is 30 ft, they are 5 turns of dashing away, although that assumes we're not pushing them another 300 feet out to 600 feet or 1200 feet away with distant spell.
The main problem this build would have is still being able to see the people you're supposed to be fighting. This build is fantastic if you fight on bridges or near cliffs, despite the once per turn clause you can move three people quite far on one turn.
You could attack someone you've hit last turn with the curse on them with your first blast. moving them 25 ft.
You could attack someone new, moving the curse to them moving them 25 ft.
Then quicken an eldritch blast and move a third target (using two beams) 20 ft. away.
Is it worth the specialisation? probably not on its own but you're still sorlock with advantage on con saves, metamagic and all sorts of other tricks. I mean you can breathe underwater at level 1, and if it's a nautical campaign you're going to push a lot of people into the ocean.