Fish are like people...inherently lazy. When possible, they follow the path of least resistance. Both in the ocean and in large rivers, they rest in back eddies where the current is slower. In the ocean, when currents are strong, fish on the backside of islands ... if you can find a spot, that is, among all the commercial boats! Waiting for a rising tide before entering a river, makes their up river travel that much easier. Its not always possible to wait but if given a chance or if the urge to move upstream is not that great, fish will wait for slack water. Fish know what slack water is. Even in mid ocean there are periods of slack water and predator fish take advantage of that, knowing that bait fish move more readily when currents are slower.
Having never fished the Squamish system to any degree, I don't know the time delay between tides at Point Atkinson and the mouth of the Squamish but I would hazard a guess that it wouldn't be any more than half an hour, if that. Therefore, if you wish to take slack water into account, the best time would probably be two hours BEFORE the high tide at Point Atkinson to a few hours after. Remember that the positive effects of slack water do not only occur precisely at the moment a high tide is predicted but also in the 1-2 hour period it takes for the tide to take full effect as well. It always takes longer for the tidal effects to diminish on a river so the tidal effects on the Squamish might take 5 or more hours before the river returns to 'normal'. Certainly the further up the Squamish you fish, the longer it will take for the tidal effect to occur and you will have to judge accordingly through practice and observation.
The effects of how tides affect river fishing are much more pronounced in my opinion on smaller streams. I could tell you in great detail how the tides affect systems such as the Sumas but that is not what you are asking for here.
