I like starting with double gauntlet to make into bracers/drum or urn. Double bracer really gives a punch to tidebringer during the laning phase, and adds survivability (which i never bad). Kunkka loses power late game, allowing you to focus more on a midgame build. And he doesn't have inventory slot issues, so he can keep the bracer(s) for quite a while
Lots of people level up X twice, and then stop leveling it. At level two, it is timed so you can cast X->torrent (->boat if necessary), and you should never miss.
I would mentioning the description for the boat that allies hit by it get double hp for a short time (sort of). Even if you hit no enemies, a boat that hits all your allies before a fight is a great boat.
You can score a lot of kills on fleeing enemies with well placed tidebringers. Keep that in mind when you are chasing someone down a lane and a new wave of enemy creeps are coming, or chasing someone through the jungle.
I don't like ac on kunkka, and would rather go for satanic or mkb at that point. But that's just me.