Just lighting her isn't that hard
Getting a moonless night given the state of the weather lately, isn't going to be that hard either, just use the cloud cover.
I'd go for a sunset, or just after sunset shoot instead of pitch dark. You're going to struggle to get starlight in due to light pollution (which is another thing to contend with)
Apart from safety, as the beach at night with gear isn't going to be an easy plance to work, challenges I'd expect to face are....
Model not realising how cold it will be (Tenby's not Tenerife, and certainly not Trinidad)
Needing to get in the water yourself with camera.
Probably needing to get a light or two into the water
Sort of thing she's looking for, if it's the lying in the surf looking sultry (whilst hiding goosebumps), is easily enough done with a large softbox, pretty low to the ground with you lying and shooting from underneath. Have a look through Nudes & Glamour for Jason Cole's stuff, he does lots of this type of thing.
For a less gear intensive style of shooting, and a bit of a laugh too, have a look at Tomas Whitehouse's BTS video's here for a late evening water based shoot.
http://www.tomaswhitehouse.com/2009/08/05/king-for-a-day/
Oh, and expect the mother of all retouching jobs dealing with how her body reacts to the cold water. Goosebumps are a royal PITA to get rid of.