It's not that mifi's are slower, it's just that phones require less data transmitted than laptops and tablets, so streaming apps will go faster with less of a data requirement. Apple's Facetime is faster then Skype
Unlocked iPhones are GOLD on ships. If you get one, you can easily resell it onboard in case you quit ships, and most likely for more than you paid for it.
Something else to add to the mix . . . I'm right now in Grand Cayman - my tablet is on the restaurant wifi, but my iPhone is on the Boingo network - if you've traveled, that's the same network you see in airports. I pay either $5 or $10 a month for a 2GB/month subscription and most any port the mainstream cruiselines visit there will be a hotspot
If your daughter's old enough to type - look into gmail and their email chat client. Most likely you'll have Internet (though probably limited) on your office computer, and can at least chat there. BTW - ships will 'throttle' or limit anything that streams or is considered 'social networking', like Facebook or LinkedIn as crew will sit there for HOURS and hog bandwidth that could be used by paying guests.