Good morning and welcome, Karl,
As you might already know, we have only two windsurfing outlets in Melbourne,
www.shq.com.au/ (closest to you) and
www.rpstheboardstore.com/ (another 10 minutes' drive)
You might consider going to a newsagency and getting a Melways street map. The locations that follow are cross-referenced to the Melways.
Frankston Lifesaving Club, Melways 100A, B6. A known car-breaking-in area. Works in northerly and southerly.
Frankston, bottom of Oliver's Hill, Melway 102, A4. Works in northerly.
Mornington, Melways 104, D10. You need to explore this area for sailing spots or get advice from "Forum Lurkers".
Mt. Martha, Melways 144, K11. Works in westerlies.
Safety Beach, Melways 150, D11. Works in W and SW
Rye, Melways 168, J4 (in the Yacht Club area). Works in NE, N and NW.
Between Safety Beach and Rye there are many beaches to pick from, but the two above are the most popular.
Coming back to Frankston and heading north:
Keast Park, Melways 97, D11. Works in N and S
Bon Beach, Melways 97, C4 (off Williams Gve). Works in N and S.
Parkdale, Melways 87, B10 (at Parkdale Yacht Club). Works in S and E
Mentone, Melways 86, K8. Works in S and E.
Most of the parking areas north of Frankston are "fee payable", around $3/hour or up to $12/day, so take coins with you.
As in the northern hemisphere, we are having a very odd season here. We normally get Southerly afternoon sea breezes, but these have been rare this year.
Coming from the UK, you need to be very careful in our sun from October to March, between 10 am and 5pm. Suggest use an SPF 30+ sunscreen on all exposed skin (and the top of your ears) and wear a floppy hat with a big brim.
Hope this helps.