@ChrisPerkinsDnD Can you recommend a monster that might live in a well for a level 2 party?
— sally Bidgood (@sabiddy) April 16, 2017
Monster Manual: ettercap, faerie dragon, gricks, green hag, ochre jelly, water weird. Volo's Guide to Monsters: meenlocks. #WOTCstaff https://t.co/lzmR3BVJP1
— Christopher Perkins (@ChrisPerkinsDnD) April 16, 2017
You can also just take any appropriate level creature, and fluff it to be aquatic. In xfiles they had a leechman in a port o potty.