The Keeper of the Lake

Keeper of the Lake looks to be delayed indefinitely but if you are looking for a similar vibe, Lover Merman seems to be it. Also billed as a BL with a Merman lead, this is supposed to have drama, romance, mystical lore and more.

Lover Merman is a Thailand production.
Keeper of the Lake was to be from the Philippines.

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.