Just the fishing and sealife feature from the ethereal mod:
https://forum.minetest.net/viewtopic.php?f=11&t=7656&start=150#p153693
for a simple and multiplayer working fishing and seaplants.
Ethereal by Chincow Updated and maintained by TenPlus1 Extracted with small modification by Xanthin https://forum.minetest.net/viewtopic.php?f=11&t=4375&p=154166#p154164
Note: rename the folder into "fishing", but actual the textures are missing here, so better use the uploaded file in the forum. If you want only fishing or only the seaplants, comment out the corresponding dofile in the init.lua file.