The most common answer to the clue “semi aquatic rodent” in crosswords is COYPU (5 letters).

Other possible answers, depending on the letter count and specific puzzle, include:

  • MUSKRAT (7 letters)
  • BEAVER (6 letters)
  • CAPYBARA (8 letters)
  • WATERRAT (8 letters)

If you share the pattern (like _ O Y P U or number of letters), the best match among these can be pinpointed more precisely.

Information gathered from public forums or data available on the internet and portrayed here.