Be interesting to see how well they work. Hopefully no unexpected handling issues. 

 I have only used it empty but got me thinking they look like a sealed unit with bungs 

Personally I'd leave them filled with air as buoyancy rather than filling them. should add stability that way. bungs would be to drain any ingress of water afterwards.

Cheers

They look more like sponsons rather than water filled stabilizers. But I would agree with Paul H on this one.

Happy boating

would definitely work better filled with air as the air pocket would resist being pushed under and give floatation on that side if a few anglers go to one side etc. Would be of more benefit than filling with water and having the hull sit lower as in a barcrusher type scenario. Filling with water also means you have to carry/push the added weight.

Added safety of having the extra floatation if hull fills or takes on some water etc. (and very hard to get the water out whilst on the water).

Theoretically it would allow extra weight to be carried on the boat/transom

That's all assuming they are sealed from the rest of the hull

I bought a ali penquin years back that had been modified with similar looking floatation devices welded on , It was a bloody nightmare in a following sea in fact I only kept it 3 months it was so scary and sold it on dreamers corner in karratha , guess what 3 months later it back on dreamers corner

Agree with Paul H, leave the bungs in, giving you more buoyancy.
Look like they have been put there to harden the ride up and stop the boat leaning over into the wind when travelling plus stopping it laying over if fishing more than one person from a side.

 not sealed and bungs at low point to let out water that would otherwise be stuck?

Having a look at the pics I've 2 observations and 1 question...

Observations

1) it doesn't look like they're add-ons.  These from there being no weld joint on the gunnel and no obvious join line on the transom.

2) it water was getting below deck and into what I suspect are chines then it'd be a bugger to drain so some clever person has added bungs to drain any water build-up in this area.

Question

Do you know that they are sealed units below deck? Have you had a look through an inspection hatch or similar or just assuming?

From my perspective, until you've answered the questions I'd not be filling with any kind of water ballast else you might be creating a significant problem for yourself if the weight of the water shifts for any reason.

Let us know how you go.....