The fish-like alien has psionic powers. When the lost astronaut enters the cave, the alien makes her believe she is seeing herself in the pool. This allows it to get close and suck her mind (and body) dry. The alien then dresses in her suit and starts walking, using the compass to find the other astronauts, just as the lost astronaut was doing. My guess is that it use its psi powers to pass itself off as their lost colleague (it knows her memories now, as well as her form). Whether it wants to feed on them or infiltrate the crew we don't know. Note, the motes of light outside the cave (which follow her in) indicate that the carving is a figment of the astronaut's imagination, induced by the alien. This also goes for the pool! And if you're asking why the astronaut saw herself in the pool, well she's the first human the alien has met, so that's the only image it could send into her mind. And the alien was naked, so she had to be naked too.
Review of The Sea of Perdition
