Really good watch, could watch again, and can recommend.
Even after 25 years, this holds up, though a modernization is probably due soon.
Hop in the way back machine and see some of the greats early on: Tom Hanks, Daryl Hannah, John Candy, and Eugene Levy. That team pretty much carries the movie and they go through a wonderful, fun-filled adventure into a mystery of a new girl in Allen's life. Honestly, you could probably cut out the mermaid bit and have her be a crazy foreign girl, though that sounds like a movie someone has made already.
It being a mermaid movie definitely makes some allowances, and makes the movie more memorable. It adds in a metaphor of the importance of communication into the writing, and gives a focal point for the story to move around and become a focus in the 3rd act. As there is barely a direct antagonist in this, this sort of conflict is usually how a romantic comedy resolves.
It's a fun watch, well made for it's time, and imagine most people, especially mermaid movie fans, will enjoy it.