what effect does the tide in the river at sandy point have on gps speed ???

None.The Maccquarie guys had to measure the current for their record attempts and found that there was a 2kt influence but because we run very close to the bank there is none

When the tide is slack, there arn't any standing waves, so bugger all chop. Which implies that when the tide is running, we get what we call the whoop-d-doo's at the end of the speed run, which can be up to about 30-40cm on a rough day.And often an incoming tide gives a bit more chop, than outgoing.