How to get wolves across an ocean?

A tamed wolf will teleport to you if are out of his reach. However, a tamed wolf will not teleport to you if:

• It has been ordered to sit.
• You are not touching the ground (flying, using elytra's, using boat's).
• It is in a mine cart or boat.
• You go trough a portal in which case it will wait for you to return.
• It is in a chunk which unloads, where it will wait for you until that chunk is loaded.

The best way is to use a Lead on it and then go slowly with your boat. Your wolf will be forced to follow you.

My personal favorite way is to use boots enchanted with Frost Walker. Frost walker makes a path of ice under your feet when you walk across water. Tamed wolves will follow you across the ocean on the ice.

Be aware that it does melt rather quickly during the daytime, so it's easier to go at night, especially if you're planning on stopping for very long while you're crossing the ocean.