That really sounds like the floodgate was never actually built - a dwarf brought it over then dropped it on the ground. Which suggests you connected the lever to some other floodgate or something entirely different. Do check: the 't' view can show whether it's been built or if it's just an item on the ground.
1: stockpiles for food and the other stuff in the wagon
2: carpentry and masonry workshops
3: bedrooms and a dining hall
4: an underground farm
5: a kitchen and a brewery
I've made stockpiles for cloth, leather, food and finished goods, but when I breakdown my wagon I still see items on the ground... but no dwarves have moved them anywhere... is there another type of stockpile I should be making to get everything from the wagon inside? So far I've just embarked with the default items.
Steps 2~3 and 4 can happen at the same time. Brewery is much more essential than a kitchen. As for items left on the ground, why not loo'k' at what they are? That should tell you what stockpiles you want.
Speaking of which, you don't want the default items. At least get rid of the crutches, splints and maybe buckets. Also remove the thread, cloth and maybe bags as you can get those by caravan before you actually need them (for moods). Spend the points you freed up on maxing out skills on all dwarves, then spend the rest on whatever you want (food, seed, booze, wood, stone, fuel, ore)