I've played through Rise of Tiamat a few times with various DMs, and all of them had different approaches to it. The first questions you need to ask yourself is: "Do I want PC death in this campaign?" and "Is PC death a bad thing?" Let's keep it mind that going against a cult backed by dragons and potent casters is in itself fairly deadly, and no fight with a dragon should be taken lightly. Now if you may want to prevent player death for several reasons, but keep in mind that only sending the party against encounters that they definitely can win dulls the excitement somewhat.
Now, should your concerns remain, consider talking to your party before the start, or shortly after the beginning, of the campaign. Explain to them the severity of the situation, and that not all fights are meant to be won. Sometimes, the right strategy in a fight will be to flee, and even then that might not work. Encourage them to plan ahead, and if a character dies take a break after the game to discuss the consequences of this death on the party and their state of mind.
Now, to address the actual questions:
1) Assessing party strength and capability: Given the various different compositions a party can take, it can be hard to accurately pinpoint how difficult a certain fight will be for a group. The best way to give your group a preview of the fight is to pit them against a similar, but weaker, enemy first. Like a vaccine does to an immune system, this precursor to the actual challenge will allow the party to formulate a strategy and learn of their own strength and weaknesses against that type of creature in order to adapt appropriately. Likewise, research will let your group find out more info about their foes. Consider having them talk to a professional dragon hunter or a draconic loremaster for more information. If they encounter a town ravaged by the dragon they're currently hunting, or the wreckage of another ship, in your case, they might get a better sense of that monster's destructive potential.
2) Can 5 level 10 characters take down an adult white dragon: Now that heavily depends on group composition, and with the given info it's extremely hard to assess. Needless to say, with bounded accuracy technically a group of mid-level characters can technically take down almost any high-level monsters if they roll well and the monster rolls poorly. Also, if your group is a gang of multiclassed, optimized badasses, they might have no problem taking it down. That said, the encounter should be a hard or a deadly one with that kind of group. The short answer is: maybe. The long answer is maybe as well, but with a lot of needless explanation attached to it. But level 10 characters have access to spells to scry and bring back the dead, which will allow them to prepare, and lessen their losses as well.