And yet there's a reason a helicopter looks somewhat like a bug and a glider looks somewhat like an albatross.
The clever designer of the tuna fish (was that a French guy or some Italian ?
) made a creature that is fast, powerful, efficient and maneuverable in spite of having a fairly rigid stocky body. The delicious fat torpedo of the sea!
The hydrofish looks like
a tuna to me, just not a static profile view. More of an adaptation, of a swimming tuna's shape, from an angle. Kind of like the angle of a hydrofoil while riding.
Tuna generate their power side to side, and much of their body is pretty rigid, as fish go, so a heeled over tuna shape and a hydrofoil seem to have a bit in common.
Flow condition, mechanical stress, etc. produce many examples of convergent evolution. Although the hydrofish has nice carbon "scales" I bet its shape may require less carbon than some others.
You could be a cynic and call the hydrofish a gimmick shape, but there are a lot of things about it that are brilliant, at least as a logical starting point for evolution to suit the task. The obvious first change being a serious diet, as the thinner hydrofoil gets its power through the mast not muscles, and next an asymmetry of the lifting profile...
I was only half joking though when pointing out it also looks like a way get the attention of a much bigger fish. It would be an interesting proposition to ride it out the golden gate at dusk, I think I will let someone else try that.