Ok, i've marked some pics up. Black circle is general sailing area, yellow circles are flat(tish) water

Safety bay: Weed fin generally required, but if you can live with towing some to the sandbar (to penguin island) with you, its not too bad over the sandbar itself. rig at carlise st boat ramp, which is the western most driveway to the beach between the two yellow circles. Try to park on The roadside as the rangers are nazis about the trailer parking. If you want to try the pond (eastern yellow circle.) rig on the grassed area just west of bent st boat ramp.

Lancelin: Weed fin generally not required. everyone rigs at the surf club near the southern point as most are there to sail waves on the other side of the reef. but if you want to sail the flat water, i'd rig up closer to the tavern (the driveway to the beach at the northern end of the black circle) to give yourself a bit more space to sail the seabreeze without hitting the reef. Keep in mind that its an offshore breeze sailing this area.

Cervantes: Weed fin generally not required. rig up at thirsty point which is the point on the north end of the black circle. Alternatively you can follow the dirt rd south just out of town, and it'll lead you to the beach right where the southern flattish water spot is and rig there, the beach sand is fairly compact so you can drive onto the beach and rig there. (At your discretion, the sand will find every crevice in your vehicle when the wind's up). Keep an eye out for the occy pots everywhere: two small bouys, generally on the verge of sinking with all the weed growth (aka camouflage).
theres also some nice flat water on the north side of town, but it can be gusty, being sheltered by the town and all. (Not to mention it's offshore on a seabreeze.) i wouldn't recommend confining sailing to directly west of thirsty point as it confused chop. Sail straight upwind into the bay.
the water up there is crystal clear and it can be deceiving. It looks like extremely shallow water In some spots but provided you stay over sand youre generally in over a metre of water. Don't sail over dark spots (reefs) because they are generally shallow. Its great fun sailing out in the bay at cervantes, but i wouldn't call it the flat water you're after. Head straight to the yellow circles if you want flat.
or alternatively, just go straight to shelly beach