Remember, you are the GM. If a player is trying to make a declaration that you think is unreasonable, you can always inform them thus and not worry about it. This is why, for example, no matter how high a character's investigation is, they will not be able to find military grade weaponry loaded with garlic-cold iron rounds leaning up against a wall in every scene.
That said, declarations are your friend. Yes, even as the GM. It does a few things for you, all of which are good. Firstly it allows you to use brevity when describing a scene, trusting that players will fill in plausible and necessary details as they become relevant. It also draws players into the game as authors of their own fate rather than just observers; it gives them a feeling of creativity, spontaneity, and control. Finally and most deviously, it conditions your players to accept details added in the middle of a scene, which gives you a lot of flexibility as a GM.
To illustrate that last point, let's take your bucket example a little further. The players are fighting a sorcerer in an office building, and someone has set the hallway on fire. Thinking quickly, one of your players attempts to make the declaration that there is a bucket nearby filled with water. "After all," he says, "it's after hours, so the janitors were probably out and about until we started this fight." Deciding his declaration is plausible, you set the difficulty at 4. He succeeds, and uses his newly found bucket to douse the flames. Now it's the sorcerer's turn, and things aren't going well for him. He needs a way to bug out and prepare for round 2 with the PCs later, but you're not sure if he'll be able to escape on his own. Bingo, declaration time! The bucket was there, so you have the sorcerer basically use a declaration to find the janitor who was using it, and all of a sudden he has a hostage! Now in another game, your players might protest and demand retroactive alertness checks, say their characters would have found the janitor and so on. In DF, they know how declarations work. Hell, they just used one themselves. So they are conditioned to accept this unfolding drama, giving you a lot more space to play in.
If you are still leery about declarations, I think it is reasonable to tell your players that the default assumption in your game is that every declaration is untenable until proven otherwise. It is their job as players to convince you of the plausibility of their declaration. Thus "I find a bucket," might be an unacceptable declaration, but "Since we saw the night janitor moving around before we got in this fight, I will use a declaration to attempt to find his water-filled bucket," would be much more likely to pass muster.