Breaking waters: "Anchor and Hope"

There are several reasons to feel fondly towards the disarming indie Anchor and Hope, shot around London by the Spanish writer-director Carlos Marques-Marcet. It's one of very few films to make attractive use of the capital's canal system (and, in an eccentrically novel touch, to linger on those canals' uterine properties: you'll never look at lock gates in quite the same way again). It's one of those multilingual co-productions - completed with EU funding - which will likely disappear after next March; its worldliness would seem more than ever an endangered species as we enter the era of Downton movies and Johnny English sequels. And there's a breezy warmth coming off the screen, with which you could fool yourself the British summertime isn't over yet. All this has to be set against the fact that, for at least an hour, what we're getting is really no more than a sitcom crammed into a barge moored somewhere around Gasholder Park - a sort of Man About the Houseboat. Having put down roots of a kind, butch, impulsive Kat (Natalia Tena) and sensitive femme Eva (Oona Chaplin) are floating ideas for the next stage of their relationship. Eva wants kids; Kat does not. A potential solution to this impasse arrives in the form of Kat's aptly named Spanish pal Roger (Euro cinema's shaggiest new pin-up David Verdaguer, the dad in Summer 93), who presents as a willing sperm donor to just about every woman he meets. Thereafter, it's a matter of these characters rearranging themselves within some especially narrow confines, although the script - written by Marques-Marcet and Jules Nurrish - throws in one second-hour curveball that really does rock the boat.

It's very clearly the work of a close-knit cast and crew messing around by the river: Chaplin's mother Geraldine is roped in for two scenes as a former flower child bearing cautionary tales from the counterculture and lecturing these kids on the limitations of alternative lifestyles (rather pointedly, the character goes under the name of Germaine), and matters proceed in extended, sometimes rambly semi-improvised scenes in which the central trio thrash out the finer points of their futures together. (A real sitcom would have a script editor aboard to pare back the indulgence, and hone the dialogue to its funny essence.) Still, it should work so long as you're prepared to travel alongside these goofy, charming performers, whose gameness - their willingness to go with the ebb and flow of any given development - fits the characters to a T; this being the case, you'll happily follow everyone into the more dramatic second half, wherein it suddenly becomes apparent there are stakes (and hearts, and lives) in play. Its ultimate destiny may be to serve as a watermark in academic studies of how film and television have responded to the rise of non-traditional family units: you can't miss how relaxed the film is around its own set-up. In any previous decade, sperm donation and same-sex parenting would have served as the basis for farcical conniptions or deeply conservative horror stories. Here, they're just developments people muddle through, figure out, get on with. Its relative ordinariness - the calm and steady hand Marques-Marcet keeps on the tiller - is a big part of its appeal.

Anchor and Hope opens in selected cinemas from Friday.