After Scott wrote about his concussion incident, I looked into multi-sport helmets that would cover bikerafting, possibly with some rockfall-risking activities added. Each activity poses a different problem in design, with safety standards setting the targets. Each standard requires managing energy absorption differently. For whitewater, the helmets seem mainly to protect against multiple low speed horizontal hits; for bicycles, moderately high speed single hits; and for rock climbing/mountaineering high speed rock fall. And there is the matter of having lots of cooling vents for biking, not too many holes on top for rock fall too enter through, and flow control for water while swimming.
I found only one helmet that meets the European standards for all three. The helmet is the Kong Scarab. It’s light enough and readily available from web-based retailers, if moderately expensive. IIRC, no other helmet even covered the spectrum of bikerafting risks. For straight whitewater I might choose a good multi-hit whitewater helmet, but multi-hit helmets weigh too much for a serious walk, in my opinion, and are not safe for bicycling use. The tradeoff for lightweight is that the helmet gets replaced after a hard hit.
You can see the helmet on the manufacturer’s site in black, but I have seen it for sale on the web in red and white.
I ought to add that I found a comment on one site, perhaps by a manufacturer’s salesman, that suggested another manufacturer was planning to compete with the Scarab. However, the comment is now stale, and nothing surfaced. Vaporware; apparently providing evidence of the difficulty of designing a good packrafting helmet.