Looks like this tweet suggests bigger boats are coming to the Minecraft Pocket edition. Hopefully we can get answers soon to see if this will be coming to all the other versions as well!
I guess it's getting there pic.twitter.com/kGD0ASocKj— Tommaso Checchi (@_tomcc) January 26, 2015
That looks pretty interesting! Atleast if we're talking what fits in boats and not what you can fit in a pig. Tommaso, a MCPE developer goes on to tweet:
Actually it might be that there's no way to make a second entity fit in the boat without making the boat huge, we'll see.— Tommaso Checchi (@_tomcc) January 26, 2015