It depends where abouts but usually a light offshore wind will equal flat water. Long point is quite flat with the common south westerlies, I think it's becoming quite popular for jet skiing. I never really go to rockingham foreshore but I think that stays quite calm as well.
A southwesterly as said by Revok can be good if light. As it swings more around to the west or wind strength picks up above 15kts and can get pretty uncomfortable.
The jet ski and water ski area is not too big, but it does border right on the area where the western courtesy moorings are and can be a little bouncy due to the inconsiderate skiers (not all)
I often go to Mangles instead of following the crowd to Rotto. The moorings have a free 72 hr limit, the foreshore is just as nice as Thompsons Bay and there are cafes on the foreshore are reasonably priced.. Downside is you have no protection from anything from the S/W to N/E.
The good thing (probably the only thing) about Rockingham is that it's surrounded by water on 3 sides.
This means you can get flat water pretty much any where as long as the wind's not due West.
Safety Bay and Warnbro sound on the morning easterlies is flat and very protected as well as Shoalwater Islands Marine Park.
On the seabreaze the Rockingham forehore / Palm Beach is protected and flat close to shore.
