The big problem is heat rejection. Cellular biology has similar constraints as heat engines, so for energetic creatures there needs to be a way to dispose of waste heat. At the temperatures of boiling water, proteins actually start changing form or denaturing, which is why a raw egg looks and behaves differently from a hard boiled egg.
For a fish, this might be dealt with by having a small surface to volume area, so the fish does not absorb too much heat, and having some sort of organ or structure which acts like a heat sink. When the fish approaches the maximum heat load it swims away from the volcanic vent and radiates heat into the cooler waters. Such a fish might look more like a beach ball or rugby ball than what we think if as a fish.
This could provide some interesting adaptations; the fish might have extensible structures like fins which act as radiators in the cooler water, shedding heat faster. Predators might lurk near the thermoclines, searching for mobile hot spots which allow them to zero in on prey. Creatures might even evolve like giant squids, with tentacles or similar structures to "fish" for prey or edible materials from the cooler areas outside the vents.
So in terms of illustration and design, you need to think of how the environment will affect the creature, and what sorts of adaptations will be needed to deal with them. A plump, football or rugby ball shape provides minimal surface to volume, limiting heat absorption. Large size actually helps since the interior volume increases as the cube of the size, while the exterior area only increases as the square of the size. The large interior mass could act as the heat sink. Once it swims out of the hot zone, it spreads large fins to radiate the heat (or maybe elaborate structures like a lionfish), but will have to contend with potential predators evolved to seek out hot spots in the water. Spines, poison, powerful jaws or even sheer size are all common defense mechanisms in the wild. Poisonous creatures often have very bright and distinctive colour schemes to announce to potential predators that they are poisonous.
Square Cube law
Large size vs predator
Lionfish. Large extensible fins, poison spines and bright colours all in one
A giant squid. This creature could lurk around the edges of the hot water vent and reach in to grab prey
So think about the environment, and have some fun with it.