It's a Kryptonian bot
Unfortunately, it's too early to provide a screenshot or script direction, but I've made a careful note of the scenes involving General Zod's body and the birth of Doomsday within the Kryptonian ship.
Ignoring the Kryptonian hover-bot floating above the liquid that Zod is lowered into, and also ignoring the various cables / tentacles that descend from the ceiling of the chamber that seem to wrap around Zod's body, I can confirm that there is something swimming in the liquid.
It's quite clearly another Kryptonian bot, oriented horizontally rather than vertically. The length and width are identical to that of the hover-bot and it has an identically-shaped upper half as the hover-bot. But its lower half consists of approximately eight appendages that permit it to flagellate through the liquid. (The flagellation gives it an impression of being alive.)
^ Has the same top half as this but a different bottom half...
NOTE: This is un update of a previous answer. The original answer was written after a single viewing of BvS. I've updated it more conclusively after a second viewing. (Some might question the wisdom of seeing it twice, but what's done is done.)