For roaming charges, check here: https://giffgaff.com/top-up
iMessages require in internet connection.
- If you send / receive iMessages using your mobile data roaming, then you will be charged accordingly corresponding to the size of the message (e.g. 1 MB picture = 20p)
- If you send / receive iMessages when connected to WiFi, then this would not cost you any extra
If you intend to use your phone a lot, then I recommend purchasing a local US SIM card upon arrival.
Have a good trip!