Has this already been posted? Don't think so.

The Royal Isshu is a boat that is unaccessable until you have the national pokedex. It is at the dock in Hiun city. The Royal Isshu sets sail once during the evening (17:00 to 19:59).
It costs 1,000 pokedollars to board the ship. However, there is a time limit. Once that time limit is up, you have to get off.

There are various trainers on the ship. Some, after beating, give you items from previous games. The items obtainable are Lava Cookie, Old Gateou and Rare Candy. Also, large amounts of money can be obtained.

On the ship, there is a place where you can watch the Royal Isshu on its journey. It goes under Sky Arrow Bridge and loops around Hiun City. This feature really shows off the games 3D rendering enguine.

Source: Serebii.

hmm, I should try that, thanks Rotom,

*JC gets his DS and changes the clock*

Oh sweet Arceus, that's some good 3-D graphics.

Hiun City looks beautiful at night.

It looks amazing!

It's a really beautiful that I've seen from the pictures.
Now, all I have to do is experience the awesome for myself~

Golly those trainers have some lousy Pokemon. The prizes look pretty nice, though, the Rare Candy especially!

They better do this for SS Tidal in the Ru/Sa remake.