Hi there,
Not sure what you have over there, but I use a 28" Samsonite F'lite:
Amazon.com: Samsonite® F'Lite 28" Upright: Apparel
and a mesh backpack (with both top & side zip) as my boat bag:
Akona Deluxe Mesh Backpack (AKB234) from LeisurePro.com
plus, a 20litre drybag for towel, clothes, logbook, lunch etc.
Both my buddy & I can just squeeze both sets of gear into the suitcase + 2 mesh bags, 2 small pelican boxes (for save-a-dive kit & 1st aid) and 1 dry bag to share. We both carry our clothes and both sets of regs as carry-on.
If we wanted to throw everything in the suitcase other than regs, we could each use one of the suitcases. The suitcase rolls well, but considering the weight of gear, is sturdiest when packed full rather than having stuff rolling around. We also use a luggage strap & cover the side-latches with clear packing tape with a tab folded over for easy removal (in case of inspection) as baggage handling is atrocious these days. We've had no problems since this is an unmarked case. Others in our group who have various scuba-branded bags have had gear stolen from their cases.
The Samsonite F'lite is also available in a 31", but since airlines are becoming stricter about size and weight, I wouldn't reccomend it.
Good luck.