As someone somewhat familiar with tides, Mr. Glasses here has made a huge mistake. It's true that the tide cycle is a 12-hour one (roughly, usually a bit longer and can even be close to 13 under certain circumstances), but that means from low tide to low tide (or high tide to high tide). If he just adds 6 hours under his own assumption, what he gets is the high tide point.
It would roughly still work if his assumption is wrong and they got there during the intertide period, which seems likely, and it could be even be less time depending on the phase of the moon.