I've had a Marlin for a few years, and it hasn't opened on me the couple of occasions I've used it, but I've seen what PTWoody described (assuming he meant the other type of waist

) happen multiple times for others, mostly with new (or near new) jackets. So I probably wouldn't be recommending one, unless they've made some advances in their design.
Haven't tried the Ultra, probably a step up from the Marlin, but from what I've seen (correct me if I'm wrong) they're a similar size to the Marlin and with the waist strap at the top, I'm guessing the bumbag would flap around a bit, just like the Marlin?
There's some much more compact ones available now. The great thing about the smaller ones is that you can wear them at the front (where they're suppose to be worn) without them getting in the way for paddling - it might be more comfortable to wear a waist pfd at the back like a bumbag (have seen it done a lot), but its not the most practical if you actually got in trouble and needed to put it on quickly!
The ones NSW Maritime provided for use at the NSW & Aussie Titles last year were much smaller than the Marlin and you hardly even noticed it was on. Not sure who manufactures them, but would be worth looking into as a better option, if they're available for sale to the public.
There were also a whole swag of options on display at BOP last year, including quite a few designed specifically for SUP. I'm not sure how many (if any) of these have made it to Oz yet.
I was lucky to be given one that was designed for SUP, from a company called MTI. Very compact (less than half the size of my Marlin) and comfortable to wear. The thick waist band is central to the bumbag (rather than at the top, like the Marlin), so it doesn't flap around. The velcro closure is at the top (so the uninflated jacket can't just fall out if the bag fills with water and the velcro comes unstuck). The pull cord is accessible from the side and it is designed so you pull the cord whilst the jacket is still in the bag, and as it inflates, it opens the velcro up for you, then you just slip the inflated vest over your head, and tie it on. Much better the struggling to unfold and put on an uninflated jacket before inflating it. Pretty sure it has a small pocket or phone/keys/gel if you weren't wearing a hydration pack at well.
From memory there were similar offerings from Onyx, Quiksilver, NRS and a bunch of others too. it would definitely be worth seeing if any of them had made it here and approved for Aussie standards yet?