Well, sort of. First off, I am assuming that since you are talking about the battery draining, the boat is not running and the alternator is not charging the battery. The voltage fo the battery will be lower than 14V, especially towards the end of the battery's charge. Second, a 2000W system will only draw 2000W in very short bursts (like when the bass hits). What you are looking for is not the peak (2000W) draw but the average draw. That can be a little hard to figure and depends on the volume of the music and type of music you are listening to. Also, a 2000W system will draw more than 2000/14 amps since the amps are not 100% efficient. That is why they get hot.
You did get all of the math right though
P=I*V (Power = Current * Voltage)
and
Battery capacity (in amp-hours) = current draw (in amps) * time (in hours)