Question:

In Mark 6:45, were the disciples rowing that boat for 7 hours?  I know it says the "fourth watch" but is that equivalent to 7 hours like I heard a preacher say?

Answer:

In the days before timepieces, people tended to divide the days and nights into three hour segments. While you might not know the precise hour, you could know roughly what segment of the day you were in. Armies used this to schedule turns on watch duty, so the segments were commonly called watches. The fourth watch at night would fall between 3 am and 6 am.

To determine how long the disciples rowed the boat, you would need to know when they left. Matthew 14:22-23 states that they left just before evening. Nightfall (evening) varies quite a bit depending on the season. We generally state evening starts about 6 pm, but it could be anywhere from 6 pm to 9 pm.

So they would have been rowing somewhere between 6 and 12 hours.