When I am abroad, or diving out of a boat in the UK (be it hard or RIB), I use a Stahlsac mesh bag. It is plenty big enough to get BCD, wetsuit or drysuit (although I'll have the latter on if it is a RIB dive), regs, fins, mask, torch, weight pockets, etc in, but when it is empty, it tucks under my seat out of the way. It also has an integrated drypocket right down the back, so handy for stashing my clothes and towel in while I dive. it also folds virtually down to nothing when I pack it into my travel bag.
If I am shore diving in the UK, I use a big plastic box with a lid that clips together on closing. It is identical to the Oceanic Gear Gulper, but only cost £5 from a local stationary place. Plenty big enough to get everything in, and any water leaking out at the end of a day's diving ends up in the bottom of the box, not your car carpet! (I've got a pick-up, so it doesn't matter, but friends with cars appreciate it!)
Mark