Surfboard problems

I want to make a surfboard, or a surfing board. I don't entirely know how you call these. I did make one, it's all good and all, you can ride it or whatever. Like a boat. Or a cart.

Information: It's hitbox is 0.8 in width and 0.1 in height. It's immune to drowning and all kinds of magic effects. It is NOT immune to fall damage on purpose. Instead of a spawn egg I made an item that does literally the same thing. It just looks better. When the entity gets hurt, it "despawns" and drops the item. The surfboard always has dolphin's grace. The only AI action is to swim/float in water. It's a custom model, if that's not obvious.

The problems:

• it bounces in water instead of staying still (like boats do)
• whenever the player riding it is submerged, they stop mounting the surfboard entity
  that's probably a Minecraft thing.
• whenever the surfboard collides a block above it - dies
  I tried to increase/decrease the height of the surfboard's hitbox, but that didn't change anything. It's probably from suffocation. Any way to disable that?

How do I fix those problems? Any help is greatly appreciated. thanks.