There's a guideline, which makes sense, with examples. Is a boat as complex as an iron pot? Or is it like a bell? Mechanically, it's probably between a bell and a lock, so worst-case scenario is DC 20.
And in my opinion you just pulled that out of thin air, neither you nor I have ever made a lock or a bell (in all probability), nor a boat (I helped make a canoe once, it was fairly time-consuming and somewhat exacting work). All I get out of their examples is the DC is logically probably higher than 5, and probably NOT higher than 20, but I'm not sure about that last part if I were to try to make the boat fancier.
So it's a Craft (boat) check, with a DC around 20. If I have Craft (carpentry), then that might apply, though the DM would likely impose a -2 penalty to the check. Note that the appropriate skill exists, even if it's not listed; there was never any intent to create an exhaustive list of all possible Craft and Profession skills. But this is consistent, and the player's should easily be able to infer the existence of the Craft (boar) skill.
This is exactly the level of abstraction as is required with any other task. Whenever you want to do something, the DM decides which skill or ability score applies, and the required DC for that check. The DC of a task has a well-defined meaning, with DC 10 being easy and DC 20 being hard.
And in 4e there's a DC chart that shows level 1 DCs to be easy at 8, medium at 13, and hard at 18. So the character says "I want to build a boat" and the DM says "OK, this is not too difficult for ordinary humans, you want a crude boat that will get you down river, well call that a medium DC Nature check" (I'm saying Nature since finding some wood is certainly a key part of this, and its a WIS based skill, common sense).
A rowboat costs 50gp, or 500sp. I need to supply one-third of that cost in raw materials, and then get to work. Each week, I make a Craft check against a DC 20. If I succeed, then I multiply my check result by the 20 to see how many SP worth of progress I've made. If I roll a 25 on the check, then I've successfully crafted the boat. The only vagary that I might not know is the exact DC for the check, in which case my best guess will still be pretty close.
Sure, but again that DC20 was determined by you as the DM comparing boat building with 2 other activities that you know nothing about, and who's DCs were set by someone who in turn just guessed them themselves in all likelihood. For all I know its 100 times harder to make a bell than a boat. Its all a highly dubious chain of guesswork, and the player, even knowing all the 3e skill rules ahead of time, cannot predict what the DM will decree with any certainty.
I'd finally like to note again that there ARE subtle differences here between the 3.5 and 4e case. For one thing 3.5 insists that the character cannot even TRY to build a boat unless he's got some sort of skill. This skill requires him to have expended a valuable proficiency slot on something at best only loosely related to adventuring, and we're not even CERTAIN which skill is directly applicable, this will have to be negotiated. At least in the 4e case any character might attempt to build SOMETHING, and in the course of an SC (which any really plot significant boat building will be) can negotiate the use of various skills depending on how he approaches the task (IE he might negotiate with farmers for wooden planks, Diplomacy, or cut trees, Nature, or steal wood (Streetwise, Thievery, Stealth), etc.
Even assuming 3.5e's skill system was written using extensive primary research into every activity governed by the different skills and formulated to produce realistic results verified against actual numbers its not clear to me it would be MORE FUN than the 4e way of doing it.