Fastest fish possible

Supercavitating Missile with Fish Painted on the Side.

Body design: Sleek metal cylinder with solid fuel rocket engine at the back and gas pump at the front. The pump ejects a bubble of gas through which the fish travels. This prevents cavitation damage.

Fins: Two stabilising fins at the back.

Top Speed: 230 mph

All evolutionary and metabolic constraints: Taken care of

---

Sarcasm aside, it's not obvious what type of answer you are looking for. There are several comments that point out how objects moving quickly though water suffer cavitation damage:

Cavitation is when the fluid gets churned up so small low-pressure bubbles are formed. The bubbles then collapse and damage whatever is nearby. There is a well known predator that hunts this way. See also the sexy version.

If cavitation can make holes in a sleek metal propeller imagine what it can do to a living animal!

The question is tagged "hard science" so these comments are entirely appropriate. Ignoring problems like this makes the question no longer "hard science" and it's unclear where you draw the line for ignoring problems. In my answer I have ignored the problem "is actually a missile and not a fish".