“fourth watch of the night.” Here in Matthew 14:25 we see the night had four watches. At the time of Christ, both the Jews and Romans divided the night into four watches, each being three hours long: 6-9 p.m., 9-12 p.m., 12-3 a.m., and 3-6 a.m. So the fourth watch of the night started at our 3 a.m. (see commentary on Mark 6:48 and 13:35). So this is sometime between 3 a.m. and 6 a.m. Since the apostles had started sometime before sunset (Mark 6:47) and it was now at least 3 a.m., so the apostles had been rowing for at least six hours to go about five or six miles.
“the lake.” The “sea” of Galilee is actually quite a small lake, only 7 miles (11.2 km) across and 12 miles (19.3 km) long, and the entire lake can be seen from the escarpments on both the east and west sides. The Greek word thalasso, lake, sea, or ocean, does not really refer to the size of the body of water, and thus has to be translated into the English “lake,” “sea,” or “ocean” by knowing the body of water that is being referred to (see commentary on Matt. 4:18).