Boats take damage when they run into things. But does it matter what it runs into? Does it take less damage running into dirt than, say, cobblestone for instance? I know soul sand doesn't hurt boats, but is there a more mundane material recommended for building docks?
Also, does it only take damage if you run into something while going fast? If you slow down and gently land at your dock will the boat be ok?
Just some important points from the Minecraft Boat Wiki that you may find useful.
If you enter sneak mode while on a boat, the boat will not break on impact while traveling at high speed.
Boats do not break when running into Soul Sand at water level, because Soul Sand has a lower top surface so that the boat merely runs aground rather than crashing, so it is useful to make docks and harbors out of soul sand.
As was mentioned in the comments, boats do not take cumulative damage from running into things. There is a threshold of damage above which it breaks up. Below that you can bump it gently as many times as you want without losing it. It doesn't look like the type of block you run into matters, just your speed. And running into something in sneak mode prevents it from breaking up at all.
