From the Travel Insider Website........
General Luggage Policies
Note that when maximum size measurements are shown as a total number of inches (eg 45") this is the total of the length, width and height of the piece. There are standard sets of dimensions that go to make up these totals (for example, 22 x 14 x 9 is the standard for 45") and if you have a bag that is an unusual shape but still within the total number of inches, you may find it being rejected. The safe maximum size is 45", in the form of a 22" x 14" x 9" bag. Some airlines allow up to as much as 55", but most do not.
Domestic and International Carryon Luggage Policy variations
International flights often have much stricter carry-on policies, particularly with regard to the weight of carry-on bags. Although most domestic airlines have no limit on carry-on bag weight, internationally, you will find that some airlines set such ridiculously low carry-on weight limits (sometimes as little as 11 lbs) that the weight of an empty carry-on bag is more than the total weight you're allowed to take with you!
Fitting all you dive gear into a guaranteed "any airline carry-on bag" might be tough to do.......
If I can make a suggestion......I would aim for a checked bag with NO scuba markings at all (a $25 Wal-Mart rolling duffle is perfect) and a carry-on bag (or backpack) for the mask, computer, regs, camera, etc.......
Based on my experience, airlines are enforcing luggage weight and size limits much more now than say 2 years ago. Of course, what did they expect when they started charging for checked bags?......folks just started trying to bring them into the cabin...
Hope this helps.......M