Your MSW forecast gives a current sea temperature for every spot we list. The data comes from a Satellite survey. The Satellite is capable of extremely accurate readings of the surface temperature – however at maximum it's capable of monitoring an area of about 1 Square Kilometer and the data we then receive is averaged over an area larger than this. For most of the worlds surf breaks this is a reliable guide to temperatures encountered in the surf, but there are circumstances where this isn't the case and there is a large temperature difference over a very small area that's lost in this averaging process.
In some areas a phenomena know as 'coastal upwelling' can cause problems. Briefly (and click the link for an article with more information) local winds cause a current which draws deeper, colder water to the surface. Because this deep water tends to be rich with nutrients the areas effected are often associated with diverse marine life. For example most recently the issues we've been encountering are with South African locations, although many other coasts are affected. The problem is that this upwelling is very very localised and, although accurate over the general area, our sea temperatures miss this effect.
Unless your beach is part of a local monitoring service (ie. an automated station or Met service employee is taking a daily temperature) there's nothing much that can be done to improve the quality of this data. As a result in the areas affected our sea temperatures are at best a general guide. In unaffected areas you can expect accuracy to within a degree.