On the sim card option...
DS first ship third class sea year was a Matson Lines GCX (Guam-China Express) service ship and he did three voyages. His ATT Friends and family service on "his regular phone" meant no roaming charges in Honolulu or Long Beach, but that was not/is not the case for Guam, even though it is a US Possession. For Guam and the China Port Calls he bought a cheap "unlocked" phone and would buy local sim cards that he could get from someone who came on the ship in Guam and/or Ningbo (their first port of call in China) for "20 dolla". For that situation, he found that was by far the best way to go, and when it comes to things of this nature, he's pretty frugal/savvy.
When your DS/DD gets on their ship they'll have plenty of time to talk to regular crew and get "the real deal"/good advice on matters such as this. Communications with home and friends and family is/are something that is a shared desire/need by enough of the crew they seem to have no issue sharing advice and thoughts on, even if they are otherwise "crusty".