It's still early, there are 100 other higher priorities, but I'm writing this down before I forget it.
The merfruit, specifically the cooked merfruit offers a very niche buff. I think other buffs could be applied alongside the mermaid spawn prevention to make it less niche while keeping in theme with its aquatic nature.
In total, I propose that the fruit offer 3 buffs simultaneously (I know, that sounds crazy broken, hear me out), all with the same duration to keep things easy to track.
First, it keeps its current mermaid begone buff as it is.
Second, it would allow the pirate to breathe underwater for the duration. Ship wreck a little too deep underwater, don't want to surface while you're on your way to board, does the sound of your pirate running out of air make you nervous: The solution is here!
Third, this is the big, scary, could break the game if not balanced right buff (but I think it can be done). I propose a swim speed buff, not a big one (under no circumstances should a pirate able to outswim a ship with sails fully down, regardless of wind). I'm leaning between the speed of a shark pursuing a player (probably too fast) and a galleon fully against the wind (matching that speed, not outpacing it.) The speeds mentioned above would be for sprint swimming, any buff to default swimming or swimming while holding treasure, I would leave to Rare's decision.
This would set the fruit as a must have for any business in the water and make the choice between buffs and healing foods a little more difficult.
Anyway, we're only a couple weeks into Season 14, plenty of time for the merfruit to shine as people find new uses for it. It may very well prove perfect as it is. These are just the ramblings of an old pirate.
Safe Sailing