Why? " Stayed" is given as an alternative for estuviste. Does it just mean " you were" and in which case why should "stayed " be given as an alternative?
Not usually. Being "at" a boat is like being in the same location as the boat, but not in or on it. If you are standing next to a boat, then yes, you are at the boat. However, if you are walking on the deck of the boat, you are either in or on it.