I think the term in English is "watercrafts".
Yes, except that plural of watercraft is watercraft. They should also accept "water transport" and "water vessels" here. It is not normal usage to say water vehicles.
Is adalah necessary or not?
I would also like to know this. I'm pretty sure we need a word there, but could we replace 'adalah' with 'itu'?
What about ada instead of adalah?
I don't know but here is a link to Duolingo articles on ada and adalah. Note that in the adalah article a user asks if ada and adalah are the same, and the expert (Rick392366) replies that they're not.