Nate, I would never collect on an outgoing tide, near a drain, or after rain.
No boats are usually in a couple of spots (private beach in Beverly) or if there was heavy boat traffic I would not collect. I plan on noting all the conditions on each tank I fill up. I just got back from getting NSW for my tank, I only got 150 gallons because I saw a boat heading my way in the distance so I shut down the pump and left, before there was a chance of anything getting into my water, no boats for 15-20 mins prior to the start of collecting.