What does 00-50 mean on the sectional chart under PHNL (see picture)?
That is the elevation and length of the longest co-located water runway. Elevation of 00, and runway length 5000 feet.
This information is found on page 16 in the current Aeronautical Chart Users Guide, which states:
Runway length is shown to the nearest 100', using 70 as the rounding point; a runway 8070' in length is charted as 81, while a runway 8069' in length is charted as 80. If a seaplane base is collocated with an airport, there will be additional seaplane base water information listed for the elevation, lighting and runway
In this particular chart of Honolulu Int'l, the first line of airport runway info indicates 13 L 123, which is a 13 foot elevation, lighting in operation from sunset to sunrise, and 12,300 foot longest available runway.
The next line down, which you are asking about, is the "additional seaplane base water information", described by the ACUG.