As a driver, I agree with this. While I feel comfortable in the water, I'm not dolphin. I can try really hard to be streamlined but I still have a lot of lumpy bits I have to drag around so that I don't die. Lots of drag. That's not even taking into account millions of years of evolution.

But we should also not underestimate the ability of the enemy to train dolphins.

It would be hard to send an strike team of just dolphins but I imagine a strike team of both humans and dolphins trained to fend of the defending dolphins and protect the divers would stand a chance.

AFAIK dolphins are never used together with friendly divers against other divers as the dolphins can't discern between friends or foes.

Just make sure the enemy team wears red diving suits and the friendlies wear blue ones. Foolproof!

How does the enemy get the trained dolphins there? And once discovered, an attack, dolphins or not, isn't going to have much success.

Except the divers are probably Marines. Enemies which are probably well trained and armed. No Dolphin will bump on them and place a device on their back...

You have evidently never dived. Even the biggest, roughest, toughest marine diver would be outclassed by a juvenile dolphin with regard to mobility. Let alone a trained one who didn't want you to touch it.

You assume too much on Internet. Dolphin isn't really known by their fine work with their "mouth". I'm wondering how it would place a buoy with their mouth on somebody's back..