Slam your boat into shore, destroy it, recover all materials. If it is slightly deep water, make some ladders and jump down to it.

they are supposed to but if you're in multiplayer with other players nearby that has caused them to not drop all items in the past. I don't know if that's been fixed. It was definitely a thing back at launch

All of the resources drop at the mast, so if you're standing there when it breaks and you have room in your inventory all the parts will go to you without touching the water first.

Some things sink, so if you destroy it in deep water it won't.

My boat was destroyed a few days ago and I was able to recover all resources invested in it.
Including the bronze nails.

I am not sure about a longboat, but what i typically do is sail my karve full speed onto a sandy, shallow, non rocky shore, preferably a meadow or plains biome (if you are up for it) and beach it there and then chop on the mast till its destroyed. I get all materials every time for me.

Yes from my experience but nails sink