Edit in light of your premise altering updates:
Humanity Would Never Back Down
Releasing dolphin "captives" is an interesting idea, and probably achievable, just not by going to war. Keeping humans out of the ocean however, is unacceptable - we have too much to lose. There is no way in which humanity would be bullied out of using 75% of the Earth's surface by Flipper.
Dolphins armed with the weapons you describe could indeed hurt us if they attack just the right ships/oil rigs. They could cause billions of dollars worth of damage, not to mention the loss of human life. But the second humanity caught on what was happening it would be all out war, and it wouldn't go well for them.
Their Biggest Weakness
Dolphins are mammals, and as such they need to come up for air. They may be great swimmers, but he who rules the skies rules the battlefield, and dolphins sure as hell can't fly. Aerial patrols launched from carriers would be able to hunt groups of them down with zero casualties. We would annihilate them.
The Ace Up Their Sleeve. Flipper. Whatever.
Dolphins have way too much to lose by going to war with humanity. Blow torches won't help them when they have a fleet of helicopters and tactical bombers dropping depth charges on their families.
However they do have one huge advantage: they are universally loved. What does this mean? Instead of going to war, they should go public. Rather than asking the aliens for weapons, they should ask for translation machines.
Imagine the reaction when the first dolphin ambassador is interviewed on international news, speaks of the many injustices against his kind, and begs the world for help. It would change the world forever.
By revealing that they are sapient the world would suddenly become their oyster, because dolphins are cute. Political campaigns would revolve around how each party/politician is going to help dolphin kind. Companies would be tripping over one another to provide them with services. Dolphin fashion. Dolphin hotels. Dolphin restaurants. Dolphin smart-phones, and wearable tech. Dolphin assault rifles, for crying out loud. Government agencies would try to recruit them. Foundations would grant them honorary memberships. It would be a media frenzy.
And any poor bastard who has so much as poked fun at a dolphin before would be crucified by society. Fishing companies would have to step very carefully, or risk going bankrupt - if their boats are simply not torched by an angry mob.
So why would dolphins go to war? They have so much more to gain by being on our good sides.
The dolphins would keep quiet if they knew what was good for 'em.
Let's be serious, what could a few thousand angry dolphins DO to harm humanity? Scare some fish away? Knock over a couple of small boats?
Sure, they might kill a few people, or harm some communities, but once we realized what was happening we'd make sushi out of 'em in no time flat.
If they got super sci-fi weapons from the aliens then things would get interesting, but at that point humanity would be fighting the aliens as much as the dolphins, which really changes the scope of the question.