Does my parcel from the UK fly to America or is it on a ship?

It will fly. This is usually how long airmail letters take (and in my experience they DO take a week). The actual flight is of course 8-11 hours depending on where in the USA it's going, same as if you flew there. But there's sorting, getting your parcel to the airport, Customs processing at the other end (you put a Customs form on it, didn't you?), USPS delivering it once it's out of Customs, and that all adds time. You go through Customs when you visit another country outside the EU, and parcels are no different, except they don't get duty-free!

Yes, a boat can go that fast. A ship will take most of a week (ocean liners will get you across the Atlantic that fast if you can afford to do it the grand old way!) BUT again there's all those extras to add, plus actually waiting for a cargo ship going that way, and it usually ends up taking well over a month. Just for one thing, send it by ship and unless it's going to somewhere near a major port, USPS still needs to carry it from there by train or truck.

Just out of interest, when Michael Palin did "Around the World in 80 Days", he found out that there are container ships that get round the globe in 63 days flat. But as he said, you only see water on 56 of those, so the challenge was to do it the way Phileas Fogg did in Jules Verne's book, or as close as possible to it, so it would make an interesting TV series. He made use of a container ship from New York to Felixstowe and that took most of a week.

it is by air. boats are slower.

Parcels to America normally go by aeroplane so there you go!!!???!!!