If I'm playing a Sorcerer/Warlock multi-class and I cast eldritch blast with the Repelling Blast invocation and I have Curse of the Sea from Sea Sorcery how far can I push foes away?
Assuming that these do work together you can push them between 16 and 25 feet away from you
Let's assume you hit a creature with a Repelling Blast eldritch blast which states:
When you hit a creature with eldritch blast, you can push the creature up to 10 feet away from you in a straight line.
Note that this allows you to push them anywhere from 1 to 10 feet away (or less than 1 foot, I suppose).
Then the Curse of the Sea feature states:
[...] 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...
Forced Movement. If the target is moved by your spell, increase the distance it is moved by 15 feet.
This feature requires that your target actually be moved by your spell and it flatly increases the distance pushed by 15 feet, no other distance. Thus you would only gain the +15 feet if you actually pushed a creature with Repelling Blast.
Because Repelling Blast pushes between 1 and 10 feet, the total amount you push a creature would be between 16 and 25 feet.
There is a possible issue as you level up and eldritch blast gets more bolts. The curse will not end as the spell is a cantrip, but then the spell also says that you can do this "once per turn when you cast a spell", whether this allows you to activate the curse multiple times in one casting of eldritch blast would be up to your GM as is mentioned in the related question "Does the UA Sea Sorcerer's Curse of the Sea feature work with the Eldritch Blast cantrip using the warlock's Repelling Blast invocation?"