Splash (1984)

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.

Twenty years earlier "Allen" (Tom Hanks) had a mysterious underwater encounter with a rescuer. Advance to present day and he is managing a fish market with his sibling "Freddie" (John Candy). Another trip to Cape Cod, another accident and yep, it's the same saviour - only this time their eyes meet over a choppy sea and shortly afterwards she follows him onto the land. It's lust/love at first sight and pretty soon she is living with him. She (Daryl Hannah) calls her self "Madison" after the avenue/president and after a day in Bloomingdale's television department picks up enough English to join in the rest of this rather slapstick movie - or to at last have a post-coital chat! Thing is, madcap scientist "Prof. Kornbluth" (Eugene Levy) is soon on her tail (!) and after chucking buckets of water at a similarly attired couple a few times, our lovebirds find themselves incarcerated in a lab where they are to be the subject of some nasty experiments. Can they escape the clutches of their captors and get her back to the open sea before six days expire and she finds herself stuck? The idea is quite fun and at the start when they are in their respective discovery phases, it's quite entertaining. It just repeats itself too often, though, and the presence of his annoyingly selfish brother just clutters it all up, gradually creating the look of a slightly farcical episode of the "A-Team". It's watchable but I didn't find there to be any chemistry between the stars - Hannah was never a particularly engaging actor, I found - and it's all rather forgettable.