Which of the Perth river spots has the strongest seabreeze. Pelican point or Melville or is there one that is better still?

Pelican point. Melville is good if you want some flattish water, but suffers from reduced wind strength, and it's gusty.

You can also launch from majestic point if you live south side of Melville water, and get the wind strength that pelican point gets, but there's no sand bank to end up on if something goes amiss.