I'm looking into buying a GPS for two reasons....windsurfing and long distance running. From the running side of life the Garmin Forerunner appeals.

the GPS speed surfing site says it's an allowed device but doesn't state which model of forerunner.

does anyone know if there's a particular forerunner model I should be buying or are they all good to go?

Buy the Foreunner for running by all means but it is not much good for sailing, even for casual use.

If you intend to do any sort of speed sailing it is useless. Get a GT-31 or even a second hand GT-11 for sailing.

If you want a Forrunner for personal use, I use the 305 and have never had a problem.(sailing, running and endurance horse riding)
If you want to post to GPSTC or GPSSS then get a GT.
I use both the 305 and GT in the one pouch. 305 tells me distance, time, average, max speed(only in km though). GT is set to speed so I can watch it as I sail and make adjustments to stay at highest posibble speed for conditions and targeted category(depending on ability). 305 is used to monitor distance for NM etc. and ensure I'm meeting targets.
ALL WINDSURFING POSTINGS ARE DONE FROM GT!!!
Distances on 305 are always very close to GT.

I've a 310xt, very good for running, and passable for sailing. It's accurate at a macro level (e.g. overall distance, speed over a decent distance), but not so good when looking at things like turning corners (alphas) or 2sec & 5*10.

So I'd find it great if I were just interested in how fast and far I'm going, but you need a Navi for any of the comps or if you want to analyse gybes or your speed over short time periods etc.