Sereno

same:
- both are BL
- both are about merman

difference:
- Sereno's overall mood is a bit darker than Lover Merman
- Sereno is Filipino, while Lover Merman is Thai

If you were excite for Keeper of the Lake, you might want to take a look at Sereno.
It is also a BL drama featuring a merman main Character from the Philippines.