One of my mates (admittedly a fair bit younger than you) recently took up Tris and from the same background as you- handy cyclist/runner but feck all swimming and he's doing okay: it takes a bit of practice and it's still his worst suit by miles but if you're not desperate to be winning the things then that's okay.
You get 'sprint' triathlons with fairly low swimming distances (like 400m) and in a pool, I'm fairly keen to try one of those.
You're based in London aren't you Nemo? There's one as you describe coming up at the end of April near Hampton Court - 400m swim/22km ride/5k run. Will be my first Tri of any sort, looking forward to it.
http://raceseries.thamesturbo.com/