When asked how long it's been Ben's job to fight the DNAlien takeover of Earth, Ben says "five years." Wrong. Ben's been transforming into aliens for five years; he's been fighting the DNAlien takeover for only a few months, more or less. (Hard to determine. Based on the appearances of full moons, it's been, at the very least, two months - but full moon appearances have long since proven themselves to be unreliable in gauging the passage of time in the Ben 10 universe.)
It's been established that injuries sustained by the Omnitrix aliens transfer proportionately to Ben when he returns to human form. The reverse does not appear to be true, as Ben's black eye, sustained while fighting as Swampfire, disappears completely in transforming into Echo Echo before becoming Ben again. Black eyes aren't all that swift to go away, and it seems only fair that Ben and all the other Omnitrix aliens should share in Swampfire's black eye for awhile.
Gwen's parents already know about her special powers and about Ben's transformations (ever since a HighBreed flattened their house), so it's curious, if not strange, how they could be as upset, or more, to learn of her involvement against an alien invasion with Ben.
Kevin's life's in immediate danger aboard the offshore tanker, so instead of changing into a flying alien such as Big Chill who could get there straight as a crow flies, Ben transforms into Humungousaur, who not only has to run to the nearest beach but swim out to the tanker as well. Luckily, the HighBreed Commander -- with every reason for killing Kevin -- decides merely to restrain him for a bit, otherwise Kevin'd be dead.
No explanation for how Ben's parents got aboard the tanker - or even how they knew that's where Ben was going - after fishing out an alien bazooka from their attic and, supposedly, following Humungousaur's trail.
Even though Kevin is somewhat her boyfriend, and knowing as much as Ben of Kevin's life-threatening predicament with the HighBreed Commander, Gwen decides NOT to break her parents curfew to try rescuing him (as does Ben).