True ---No reasonable person could argue that if F1, NASCAR, INDY, USAF and APBA were to rid themself of harnesses and helmets they would be as safe or safer.
But F1, NASCAR, INDY, USAF and APBA will not allow you to race with a seat belt UNLESS you also have supporting protection from the subsequent impact: ie: roll cage, capsule, canopy etc.
In car racing, you don't have cockpit intrusive water forces to deal with either, so roll cages do the job just fine.
But, if you are seat-belted in a boat capable of capsizing, then, IMO, water deflecting enclosed canopies/capsules *should* certainly be used.
I've been involved in many crashes where I was thrown from my cartwheeling craft -- had I been wearing a seat-belt ONLY and stayed with the boat (with no capsule or canopy for protection from the impact), I'm pretty certain I would have suffered more severe injuries than I did.