Pokemon name meaning:
鞠 mari – ball
マリン marin – marine (Japanese word from English)
Rill – English word (running stream)
A rill is specifically a small stream, indicating a small riverside water Pokemon (Mariru was pure water-type until Pokemon X/Y when she became water/fairy). Such reference to smallness is common in first-level Pokemon and when Mariru was named, she was first-level. Her pre-evolution, Ruriri (Azurill) came in a later generation.