How deep must I dig for a boat trench? And a few related questions.

Yesterday I dug a short trench alongside what I intend to become my docks. From the shore I dug a canal going in-land, dug one level at a time until I can't stand anywhere anymore - took a while, swimming about until your feet find some purchase, equipping the mining pick and hitting the ground once - go forward slowly until your feet find purchase, rinse and repeat.

I don't have a real boat yet I need to do a copper run but I'm not sure it will be deep enough. I tested with a raft but that's kind of useless.

Questions:

1- is it deep enough for a karve? And for later boats?

2- are there ways to dig deeper. I went to bed on that yesterday and haven't played since. Can you place building pieces underwater to stand on while you dig deeper?

3- how much variation does the tides create? I dug at (what I think is) low-tide to try and make the trench as deep as possible. Is there a risk the changing tides bump the boat repeatedly to the bottom until it breaks?

4- I see loads of very complete boat hangar posted here - are those necessary to protect your boats from the storms?