I ive in Orlando and dive in Jupiter and West Palm frequently. My personal line of demarcation is forecast 6' or higher. Was diving in Jupiter Friday and Saturday; the marine forecast moved around a bit as I watched several days prior to the trip, but the worst forecast for Friday and Saturday was 2 to 4', occasional 5'. Friday was good; on the morning the forecast had settled to 2 to 3' and that was about what we experienced. On Saturday with the forecast back up to "occasional 5', the trip out was pretty sporty...going out the Jupiter inlet especially. As expected, outside the inlet it was a little better; did two dives; was much better on the way back in.
For my own decision about whether to cancel beforehand, as I said, if the forecast on the day before has 6' or higher in it, even "occasional 6', then I generally cancel. Once I'm enroute or in Jupiter / West Palm I trust the Captain/operator. Most of the time our decisions align.