Haven’t noticed any stamina loss personally, though my experimental is a month old or so.
As for boats, I can’t help you too much. I only got a chance to use a boat recently, it was quite fun actually, unfortunately you might get Ninja attacked by a Shocker Zombie from underwater though. ( How do zombies do that anyway? )
It was a boat I found next to a Seaside Cottage, or something of that sort. My only idea is that you might need to attack more hulls to the boat.