We've traveled in the past and really didn't have any issues, but this past weekend, we had some trouble.
Babies are 8 months and are teething. During road travel they do pretty well (they sleep... thankfully). This past weekend, we took the ferry and I think they didn't get their naps in and were tired and cranky.
I guess we know for next time. Plan for naps (or stroller naps) or prepare for cranky babies.
I assume things will get worse when they get older and start fighting sleep.