You didn't mention how your son felt about the amount of time he is expected to practice so I will address this question as a system question rather than a personal one.
Life is made up of choices and trade-offs.
Many who commented about this question on the Ask PCA blog suggest that this is the case here: if you are going to excel in water polo you have to pay the price.
Nonetheless it is worth asking whether it is possible to create a SYSTEM in which one can be a committed high school water polo player without undue sacrifice of other things that make life worthwhile (being with family and friends, having some down time, reading a book for pleasure, engaging in volunteer activity, etc.). I think so, but it will not be easy to get there.
In recent years teams (in most sports) have tended to practice more and athletes have tended to compete over a longer proportion of each year's time. This can start innocently as one coach increases the amount of practice time so his team will improve. Then opposing coaches get wind of this and match and raise him.
Soon you reach a point of diminishing returns and, ultimately, madness.
If two hours of daily practice is better than one hour (and for high school I think it usually is), then why wouldn't three hours be even better? Or six?
To get out of this "arms race" you have to go up a level to rescue each individual coach from having to engage in unilateral disarmament. If one coach were to follow your suggested guidelines he would be giving his rival coaches a competitive advantage. I might encourage a coach to do this regardless but to get widespread adoption you need to address it at a system level so a coach doesn't have to be a saint to do the right thing.
The solution has to be pursued at a higher level. Your school principal or athletic director could seek agreement from rival schools to limit each schools' teams' practice time. Ideally the relevant governing body at the local, regional or state level would step in to create rational rules limiting practice time, rather than depending on each coach or school to do the right thing. The NCAA has imposed practice limits so it seems like high school water polo could survive them.
I encourage you to pursue this with your school administration. If you know parents of athletes at rival schools you might encourage them to do the same in their schools. One person crying in the wilderness is much less compelling than many voices, so organizing to accomplish your goal may be necessary. You may also want to reach out to the relevant governing bodies to see what kind of guidelines they have and talk with them about your proposal. I have found that the leaders of high school sports organizations have the best interests of student-athletes in mind so you may find support there.
I am particularly favorable to two of your proposals: limiting early morning practice time and vacation practices. Sleep research indicates that teens need a little more than nine hours of sleep per night (yes, nine!) and that they do better with a later start to school. Early practices tend to undercut these health guidelines. There are times when an all-nighter to prepare for an exam or finish a big project may make sense, but a norm of all-nighters is a recipe for poor physical health and depression. I dare say the same is true for early morning practices.
I am not opposed to selective use of two-a-days and early morning practices, for example, at the beginning of the season. I remember fondly (with the benefit of many years of distance) my two-a-day football practices in high school. It was a badge of honor for us to survive them without complaint and in a strange kind of way, they were actually enjoyable. But two-a-days as the norm is not good-for kids, families or our society.
Practice during holidays can interfere with family gatherings, so it makes sense to put some limits on practice time during holidays to ensure that families have time to spend together. With so many societal pressures that tend to diminish family togetherness, I'd love to see competitive sports respect family time.
Finally, let me say that you are not alone in your concern about teenagers' time being squeezed. Palo Alto is the center of a movement to deal with this issue.
Challenge Success was started by Stanford professor Denise Pope with significant involvement by PCA's publisher, Jim Lobdell of
Balance Sports Publishing.