If the water must be at least 3m deep to dive safely, when during the daylight hours would it be safe to dive off the dock?

Question

The water at a local beach has an average depth at high tide of 8m. One cycle of the tides takes approximately 12 hours. This periodic motion can be modelled by the function d(t)=3.5cos[π/6 t] + 4.5 where d(t) represents the depth of water in meters, at a time t, in hours. This equation assumes that the water level is at a high tide at time zero. Many people dive into the water from a nearby dock. If the water must be at least 3m deep to dive safely, when during the daylight hours would it be safe to dive off the dock?

Answer 1

Now this does not make sense. If high tide is at t = 0, then high tide is 4.5 m.

That contradicts the statement that high tide is 8 m.

Please check the post for accuracy.