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I believe they have something to do with instrument and visual flight rules - altitudes where one must fly.
Correct. From Wikipedia..."In the US, there are specific VFR cruising altitudes, based on the aircraft's heading, to assist pilots in separating their aircraft while operating under visual flight above 3,000 ft but below 18,000 ft Mean Sea Level (MSL). Unofficially, most pilots use these rules at all levels of cruise flight. FAR 91.159[3] states that any aircraft:
On a magnetic course of 0-179 degrees shall fly at an odd thousand ft MSL altitude +500 feet (e.g. 3,500, 5,500, or 7,500 ft); or
On a magnetic course of 180-359 degrees shall fly at an even thousand ft MSL altitude +500 feet (e.g. 4,500, 6,500, or 8,500 ft)."
IFR rules are more complex and account for the even numbers. Google "Quadrantal rule" for more info.
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Question is, why on a dive watch.
The Pluton was originally part of the Navitimer range and was considered a pilot's watch.Altro's excellent write up of the Pluton and its successors can be found on WUS
http://forums.watchuseek.com/f39/breitling-pluton-academy-intruder-range-brief-history-98.html
Hope this helps.