Interesting and very different. Freeoard shouldn't be a problem if it is sailed in wave conditions that scale to what a real one would sail in – I would expect flat calm conditions. When used as a Rescue Launch, they will handle a lot of weather, but then, they have a lid on to keep water out and the heavy bits are low down.
When doing the stability test, how full was the boiler? Weight of water high up won't help the ability to recover from being rocked.